<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<link:linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <link:roleRef roleURI="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLoss" xlink:href="biib-20180930.xsd#AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLoss" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails" xlink:href="biib-20180930.xsd#AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails1" xlink:href="biib-20180930.xsd#AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails1" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossTables" xlink:href="biib-20180930.xsd#AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/Acquisitions" xlink:href="biib-20180930.xsd#Acquisitions" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/AcquisitionsDetailsTextual" xlink:href="biib-20180930.xsd#AcquisitionsDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationships" xlink:href="biib-20180930.xsd#CollaborativeAndOtherRelationships" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsCollaborationsDetails" xlink:href="biib-20180930.xsd#CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsCollaborationsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsEquityMethodInvestmentsDetails" xlink:href="biib-20180930.xsd#CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsEquityMethodInvestmentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsTables" xlink:href="biib-20180930.xsd#CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CondensedConsolidatedBalanceSheetsUnaudited" xlink:href="biib-20180930.xsd#CondensedConsolidatedBalanceSheetsUnaudited" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CondensedConsolidatedBalanceSheetsUnauditedParenthetical" xlink:href="biib-20180930.xsd#CondensedConsolidatedBalanceSheetsUnauditedParenthetical" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:href="biib-20180930.xsd#CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfComprehensiveIncomeUnaudited" xlink:href="biib-20180930.xsd#CondensedConsolidatedStatementsOfComprehensiveIncomeUnaudited" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfIncomeUnaudited" xlink:href="biib-20180930.xsd#CondensedConsolidatedStatementsOfIncomeUnaudited" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/DerivativeInstruments" xlink:href="biib-20180930.xsd#DerivativeInstruments" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsDetails" xlink:href="biib-20180930.xsd#DerivativeInstrumentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsTables" xlink:href="biib-20180930.xsd#DerivativeInstrumentsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/DocumentAndEntityInformation" xlink:href="biib-20180930.xsd#DocumentAndEntityInformation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/EarningsPerShare" xlink:href="biib-20180930.xsd#EarningsPerShare" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/EarningsPerShareDetails" xlink:href="biib-20180930.xsd#EarningsPerShareDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/EarningsPerShareTables" xlink:href="biib-20180930.xsd#EarningsPerShareTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/Equity" xlink:href="biib-20180930.xsd#Equity" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/EquityDetails" xlink:href="biib-20180930.xsd#EquityDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/EquityDetailsTextual" xlink:href="biib-20180930.xsd#EquityDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/EquityTables" xlink:href="biib-20180930.xsd#EquityTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurements" xlink:href="biib-20180930.xsd#FairValueMeasurements" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsDetails" xlink:href="biib-20180930.xsd#FairValueMeasurementsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsDetails1" xlink:href="biib-20180930.xsd#FairValueMeasurementsDetails1" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsDetails2" xlink:href="biib-20180930.xsd#FairValueMeasurementsDetails2" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsDetailsTextual" xlink:href="biib-20180930.xsd#FairValueMeasurementsDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsTables" xlink:href="biib-20180930.xsd#FairValueMeasurementsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FinancialInstruments" xlink:href="biib-20180930.xsd#FinancialInstruments" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetails" xlink:href="biib-20180930.xsd#FinancialInstrumentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetails1" xlink:href="biib-20180930.xsd#FinancialInstrumentsDetails1" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetails2" xlink:href="biib-20180930.xsd#FinancialInstrumentsDetails2" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetails3" xlink:href="biib-20180930.xsd#FinancialInstrumentsDetails3" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetailsTextual" xlink:href="biib-20180930.xsd#FinancialInstrumentsDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsFinancialInstrumentsDetailsTextual2" xlink:href="biib-20180930.xsd#FinancialInstrumentsFinancialInstrumentsDetailsTextual2" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsTables" xlink:href="biib-20180930.xsd#FinancialInstrumentsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxes" xlink:href="biib-20180930.xsd#IncomeTaxes" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesDetails" xlink:href="biib-20180930.xsd#IncomeTaxesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesDetailsTextual" xlink:href="biib-20180930.xsd#IncomeTaxesDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesTables" xlink:href="biib-20180930.xsd#IncomeTaxesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwill" xlink:href="biib-20180930.xsd#IntangibleAssetsAndGoodwill" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails" xlink:href="biib-20180930.xsd#IntangibleAssetsAndGoodwillDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails1" xlink:href="biib-20180930.xsd#IntangibleAssetsAndGoodwillDetails1" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillTables" xlink:href="biib-20180930.xsd#IntangibleAssetsAndGoodwillTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/Inventory" xlink:href="biib-20180930.xsd#Inventory" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/InventoryDetails" xlink:href="biib-20180930.xsd#InventoryDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/InventoryTables" xlink:href="biib-20180930.xsd#InventoryTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/InvestmentsInVariableInterestEntities" xlink:href="biib-20180930.xsd#InvestmentsInVariableInterestEntities" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/InvestmentsInVariableInterestEntitiesDetails" xlink:href="biib-20180930.xsd#InvestmentsInVariableInterestEntitiesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/Litigation" xlink:href="biib-20180930.xsd#Litigation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/LitigationLitigationDetails" xlink:href="biib-20180930.xsd#LitigationLitigationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail" xlink:href="biib-20180930.xsd#OtherConsolidatedFinancialStatementDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailDetails" xlink:href="biib-20180930.xsd#OtherConsolidatedFinancialStatementDetailDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailOtherConsolidatedFinancialStatementDetailsTextual" xlink:href="biib-20180930.xsd#OtherConsolidatedFinancialStatementDetailOtherConsolidatedFinancialStatementDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables" xlink:href="biib-20180930.xsd#OtherConsolidatedFinancialStatementDetailTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipment" xlink:href="biib-20180930.xsd#PropertyPlantAndEquipmentPropertyPlantAndEquipment" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipmentDetails" xlink:href="biib-20180930.xsd#PropertyPlantAndEquipmentPropertyPlantAndEquipmentDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RestructuringRestructuring" xlink:href="biib-20180930.xsd#RestructuringRestructuring" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RestructuringRestructuringDetailsTextual" xlink:href="biib-20180930.xsd#RestructuringRestructuringDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RevenuesOtherRevenuesDetails" xlink:href="biib-20180930.xsd#RevenuesOtherRevenuesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RevenuesReservesForDiscountsAndAllowancesDetails1" xlink:href="biib-20180930.xsd#RevenuesReservesForDiscountsAndAllowancesDetails1" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RevenuesReservesForDiscountsAndAllowancesDetails2" xlink:href="biib-20180930.xsd#RevenuesReservesForDiscountsAndAllowancesDetails2" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RevenuesRevenues" xlink:href="biib-20180930.xsd#RevenuesRevenues" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RevenuesRevenuesByProductDetails" xlink:href="biib-20180930.xsd#RevenuesRevenuesByProductDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RevenuesRevenuesDetailsTextual" xlink:href="biib-20180930.xsd#RevenuesRevenuesDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RevenuesRevenuesFromAntiCd20TherapeuticProgramsDetails" xlink:href="biib-20180930.xsd#RevenuesRevenuesFromAntiCd20TherapeuticProgramsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/RevenuesRevenuesTables" xlink:href="biib-20180930.xsd#RevenuesRevenuesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPayments" xlink:href="biib-20180930.xsd#ShareBasedPayments" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails" xlink:href="biib-20180930.xsd#ShareBasedPaymentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails1" xlink:href="biib-20180930.xsd#ShareBasedPaymentsDetails1" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsShareBasedPaymentsDetailsTextual" xlink:href="biib-20180930.xsd#ShareBasedPaymentsShareBasedPaymentsDetailsTextual" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsTables" xlink:href="biib-20180930.xsd#ShareBasedPaymentsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPolicies" xlink:href="biib-20180930.xsd#SummaryOfSignificantAccountingPolicies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesDetails" xlink:href="biib-20180930.xsd#SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesPolicies" xlink:href="biib-20180930.xsd#SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesPolicies" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLoss" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_31EAA9F61A7FED977BE77E194FFF68EB" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_AC927956D77C52E19F097E194FFFF125" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_31EAA9F61A7FED977BE77E194FFF68EB" xlink:to="loc_us-gaap_StatementTable_AC927956D77C52E19F097E194FFFF125" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_074B4E6C949007FBF9277E194FFFA4B9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_AC927956D77C52E19F097E194FFFF125" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_074B4E6C949007FBF9277E194FFFA4B9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_36D19F14179170B448087E194FFFEBA4_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_074B4E6C949007FBF9277E194FFFA4B9" xlink:to="loc_us-gaap_TypeOfAdoptionMember_36D19F14179170B448087E194FFFEBA4_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_36D19F14179170B448087E194FFFEBA4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_074B4E6C949007FBF9277E194FFFA4B9" xlink:to="loc_us-gaap_TypeOfAdoptionMember_36D19F14179170B448087E194FFFEBA4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountingStandardsUpdate201601Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201601Member_91ACC0F56658BD1266F47E194FFF6C9F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_36D19F14179170B448087E194FFFEBA4" xlink:to="loc_us-gaap_AccountingStandardsUpdate201601Member_91ACC0F56658BD1266F47E194FFF6C9F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_6FD0346234568FE5E4907E194FFF7411" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_AC927956D77C52E19F097E194FFFF125" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_6FD0346234568FE5E4907E194FFF7411" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_8593DE29CF6C4B042A527E194FFFCA05_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6FD0346234568FE5E4907E194FFF7411" xlink:to="loc_us-gaap_EquityComponentDomain_8593DE29CF6C4B042A527E194FFFCA05_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_8593DE29CF6C4B042A527E194FFFCA05" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6FD0346234568FE5E4907E194FFF7411" xlink:to="loc_us-gaap_EquityComponentDomain_8593DE29CF6C4B042A527E194FFFCA05" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_CB7389D32CCD44779DFE7E194FFF1924" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_8593DE29CF6C4B042A527E194FFFCA05" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_CB7389D32CCD44779DFE7E194FFF1924" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_FB480A8ADE0E0E76DDC57E194FFFFA69" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_8593DE29CF6C4B042A527E194FFFCA05" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_FB480A8ADE0E0E76DDC57E194FFFFA69" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_B0CDF0F90DAE44DEE7347E194FFF2506" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_8593DE29CF6C4B042A527E194FFFCA05" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_B0CDF0F90DAE44DEE7347E194FFF2506" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedTranslationAdjustmentMember" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_1F0793F625641401E9447E194FFFB5CA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_8593DE29CF6C4B042A527E194FFFCA05" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_1F0793F625641401E9447E194FFFB5CA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_FA9BE2AF73217262ED487E194FFF912F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_31EAA9F61A7FED977BE77E194FFF68EB" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_FA9BE2AF73217262ED487E194FFF912F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_718E780E597E501A7E857E194FFFF542" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_31EAA9F61A7FED977BE77E194FFF68EB" xlink:to="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_718E780E597E501A7E857E194FFFF542" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_4DE7DA8B8DBAF9C7A6F87E194FFFBDDB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_31EAA9F61A7FED977BE77E194FFF68EB" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_4DE7DA8B8DBAF9C7A6F87E194FFFBDDB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_619D7666AA872F3C6BAE7E194FFF0252" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_31EAA9F61A7FED977BE77E194FFF68EB" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_619D7666AA872F3C6BAE7E194FFF0252" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_D4DC15FB47834AD1E3887E194FFFCCD2" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_31EAA9F61A7FED977BE77E194FFF68EB" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_D4DC15FB47834AD1E3887E194FFFCCD2" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails1" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_B221DA7B63F17301756B827D3B385DBF" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_29DF0194B898A0D4B749827D3B364280" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_B221DA7B63F17301756B827D3B385DBF" xlink:to="loc_us-gaap_StatementTable_29DF0194B898A0D4B749827D3B364280" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_9118F9CB2E6648A4821C827D3B37918E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_29DF0194B898A0D4B749827D3B364280" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_9118F9CB2E6648A4821C827D3B37918E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_F8254575878B0675CCC3827D3B37064E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_9118F9CB2E6648A4821C827D3B37918E" xlink:to="loc_us-gaap_EquityComponentDomain_F8254575878B0675CCC3827D3B37064E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_F8254575878B0675CCC3827D3B37064E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_9118F9CB2E6648A4821C827D3B37918E" xlink:to="loc_us-gaap_EquityComponentDomain_F8254575878B0675CCC3827D3B37064E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_B530E51F76B205375A13827D3B371508" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_F8254575878B0675CCC3827D3B37064E" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_B530E51F76B205375A13827D3B371508" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_F36D2DAC98123215C430827D3B37496E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_F8254575878B0675CCC3827D3B37064E" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_F36D2DAC98123215C430827D3B37496E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_E69A529F6A7D89090EF2827D3B376EAB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_29DF0194B898A0D4B749827D3B364280" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_E69A529F6A7D89090EF2827D3B376EAB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_522362612EA0A6623161827D3B3826E0_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_E69A529F6A7D89090EF2827D3B376EAB" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_522362612EA0A6623161827D3B3826E0_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_522362612EA0A6623161827D3B3826E0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_E69A529F6A7D89090EF2827D3B376EAB" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_522362612EA0A6623161827D3B3826E0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_1CC5AD042C88D3119C15827D3B38298E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_522362612EA0A6623161827D3B3826E0" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_1CC5AD042C88D3119C15827D3B38298E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_29E37200068F1F612403827D3B38D27D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_B221DA7B63F17301756B827D3B385DBF" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_29E37200068F1F612403827D3B38D27D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_746A5D3E1D29E75072D4827D3B39DD09" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_B221DA7B63F17301756B827D3B385DBF" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_746A5D3E1D29E75072D4827D3B39DD09" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_612861C50E0F8625A9C5827D3B39C076" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_B221DA7B63F17301756B827D3B385DBF" xlink:to="loc_us-gaap_Revenues_612861C50E0F8625A9C5827D3B39C076" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingExpenses" xlink:label="loc_us-gaap_OperatingExpenses_A70E24C85282DDE86AF0827D3B39BA71" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_B221DA7B63F17301756B827D3B385DBF" xlink:to="loc_us-gaap_OperatingExpenses_A70E24C85282DDE86AF0827D3B39BA71" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_403A68ECB4038A156003827D3B393EF7" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_B221DA7B63F17301756B827D3B385DBF" xlink:to="loc_us-gaap_NetIncomeLoss_403A68ECB4038A156003827D3B393EF7" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/Acquisitions" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/AcquisitionsDetailsTextual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_794BABF40D4BEDE75F84FAD0273D829F" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3F89006A96E535D0BF72FAD02739CABA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_794BABF40D4BEDE75F84FAD0273D829F" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3F89006A96E535D0BF72FAD02739CABA" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FutureContingentMilestoneTypesAxis" xlink:label="loc_biib_FutureContingentMilestoneTypesAxis_F6CB78A5833F6DA6DBA8FAD0273AFC0F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3F89006A96E535D0BF72FAD02739CABA" xlink:to="loc_biib_FutureContingentMilestoneTypesAxis_F6CB78A5833F6DA6DBA8FAD0273AFC0F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FutureContingentMilestoneTypesDomain" xlink:label="loc_biib_FutureContingentMilestoneTypesDomain_B32F8197CC2794F35C8CFAD0273A34F4_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_FutureContingentMilestoneTypesAxis_F6CB78A5833F6DA6DBA8FAD0273AFC0F" xlink:to="loc_biib_FutureContingentMilestoneTypesDomain_B32F8197CC2794F35C8CFAD0273A34F4_default" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FutureContingentMilestoneTypesDomain" xlink:label="loc_biib_FutureContingentMilestoneTypesDomain_B32F8197CC2794F35C8CFAD0273A34F4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_FutureContingentMilestoneTypesAxis_F6CB78A5833F6DA6DBA8FAD0273AFC0F" xlink:to="loc_biib_FutureContingentMilestoneTypesDomain_B32F8197CC2794F35C8CFAD0273A34F4" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DevelopmentMilestonesMember" xlink:label="loc_biib_DevelopmentMilestonesMember_CDDFEFD77E144C7CDAD3FAD0273BDB54" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_FutureContingentMilestoneTypesDomain_B32F8197CC2794F35C8CFAD0273A34F4" xlink:to="loc_biib_DevelopmentMilestonesMember_CDDFEFD77E144C7CDAD3FAD0273BDB54" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_A698066CB74DBDE300B8FAD0273C081B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3F89006A96E535D0BF72FAD02739CABA" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_A698066CB74DBDE300B8FAD0273C081B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FE53E0D55A37F26069EAFAD0273CAE3A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_A698066CB74DBDE300B8FAD0273C081B" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FE53E0D55A37F26069EAFAD0273CAE3A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FE53E0D55A37F26069EAFAD0273CAE3A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_A698066CB74DBDE300B8FAD0273C081B" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FE53E0D55A37F26069EAFAD0273CAE3A" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_KaryopharmMember" xlink:label="loc_biib_KaryopharmMember_406496F23A7140F9CACFFAD0273CFF9B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FE53E0D55A37F26069EAFAD0273CAE3A" xlink:to="loc_biib_KaryopharmMember_406496F23A7140F9CACFFAD0273CFF9B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PfizerMember" xlink:label="loc_biib_PfizerMember_B60A7DEA54D7DC768618FAD0273C5967" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FE53E0D55A37F26069EAFAD0273CAE3A" xlink:to="loc_biib_PfizerMember_B60A7DEA54D7DC768618FAD0273C5967" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TMSMember" xlink:label="loc_biib_TMSMember_924F0CD702957AAF9B05FAD0273CBC20" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FE53E0D55A37F26069EAFAD0273CAE3A" xlink:to="loc_biib_TMSMember_924F0CD702957AAF9B05FAD0273CBC20" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_AliveGenMember" xlink:label="loc_biib_AliveGenMember_B90166CABD862D26B26AFAD06486C860" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FE53E0D55A37F26069EAFAD0273CAE3A" xlink:to="loc_biib_AliveGenMember_B90166CABD862D26B26AFAD06486C860" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentInProcess" xlink:label="loc_us-gaap_ResearchAndDevelopmentInProcess_61B920F663A59C285F8EFAD0273D3B80" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_794BABF40D4BEDE75F84FAD0273D829F" xlink:to="loc_us-gaap_ResearchAndDevelopmentInProcess_61B920F663A59C285F8EFAD0273D3B80" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones" xlink:label="loc_biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones_75DBEFBB7B88800012C8FAD0273D2EBD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_794BABF40D4BEDE75F84FAD0273D829F" xlink:to="loc_biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones_75DBEFBB7B88800012C8FAD0273D2EBD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_16142365AD6987858708FAD0273D5FD5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_794BABF40D4BEDE75F84FAD0273D829F" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_16142365AD6987858708FAD0273D5FD5" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationships" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsCollaborationsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MajorCustomersAxis" xlink:label="loc_srt_MajorCustomersAxis_15DC3A1DC2D6FF130182A26C5FF04827" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:to="loc_srt_MajorCustomersAxis_15DC3A1DC2D6FF130182A26C5FF04827" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_5EF93F38A83FDE1797FDA26C5FF07CFA_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_15DC3A1DC2D6FF130182A26C5FF04827" xlink:to="loc_srt_NameOfMajorCustomerDomain_5EF93F38A83FDE1797FDA26C5FF07CFA_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_5EF93F38A83FDE1797FDA26C5FF07CFA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_15DC3A1DC2D6FF130182A26C5FF04827" xlink:to="loc_srt_NameOfMajorCustomerDomain_5EF93F38A83FDE1797FDA26C5FF07CFA" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_AbbVieMember" xlink:label="loc_biib_AbbVieMember_29A992104BB88D74D438A26C5FF053D3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_5EF93F38A83FDE1797FDA26C5FF07CFA" xlink:to="loc_biib_AbbVieMember_29A992104BB88D74D438A26C5FF053D3" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_RocheGroupGenentechMember" xlink:label="loc_biib_RocheGroupGenentechMember_DB0925869BCFC8DE9283A26C5FF0DBFC" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_5EF93F38A83FDE1797FDA26C5FF07CFA" xlink:to="loc_biib_RocheGroupGenentechMember_DB0925869BCFC8DE9283A26C5FF0DBFC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_D24AD1949E33AA48F724A26C5FF1B7C4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_D24AD1949E33AA48F724A26C5FF1B7C4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_A730B56CAF49235F66F9A26C5FF18189_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_D24AD1949E33AA48F724A26C5FF1B7C4" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_A730B56CAF49235F66F9A26C5FF18189_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_A730B56CAF49235F66F9A26C5FF18189" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_D24AD1949E33AA48F724A26C5FF1B7C4" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_A730B56CAF49235F66F9A26C5FF18189" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_150E6ABC0499B545B6A5A26C5FF1C6D6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_A730B56CAF49235F66F9A26C5FF18189" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_150E6ABC0499B545B6A5A26C5FF1C6D6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_A2D60EBA0D61AF51E11BA26C5FF1612C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_A730B56CAF49235F66F9A26C5FF18189" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_A2D60EBA0D61AF51E11BA26C5FF1612C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_2C970F2B8A2FCE658F09A26C5FF238E3" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:to="loc_srt_RangeAxis_2C970F2B8A2FCE658F09A26C5FF238E3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_F2A94BB41227E9A15941A26C5FF204BC_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_2C970F2B8A2FCE658F09A26C5FF238E3" xlink:to="loc_srt_RangeMember_F2A94BB41227E9A15941A26C5FF204BC_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_F2A94BB41227E9A15941A26C5FF204BC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_2C970F2B8A2FCE658F09A26C5FF238E3" xlink:to="loc_srt_RangeMember_F2A94BB41227E9A15941A26C5FF204BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_6D7CEAE520F5307EEEADA26C5FF267A9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_F2A94BB41227E9A15941A26C5FF204BC" xlink:to="loc_srt_MinimumMember_6D7CEAE520F5307EEEADA26C5FF267A9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_F188BD4AB86A0D5E9FDBA26C5FF28F3E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_F2A94BB41227E9A15941A26C5FF204BC" xlink:to="loc_srt_MaximumMember_F188BD4AB86A0D5E9FDBA26C5FF28F3E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_A3DA366B258E5F01DFE2A26C5FF3D7E8" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:to="loc_srt_StatementGeographicalAxis_A3DA366B258E5F01DFE2A26C5FF3D7E8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_4308E2D7A02E0B7C6DDBA26C5FF33AEC_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_A3DA366B258E5F01DFE2A26C5FF3D7E8" xlink:to="loc_srt_SegmentGeographicalDomain_4308E2D7A02E0B7C6DDBA26C5FF33AEC_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_4308E2D7A02E0B7C6DDBA26C5FF33AEC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_A3DA366B258E5F01DFE2A26C5FF3D7E8" xlink:to="loc_srt_SegmentGeographicalDomain_4308E2D7A02E0B7C6DDBA26C5FF33AEC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_US" xlink:label="loc_country_US_9178C569EA8121380AB1A26C5FF310B8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4308E2D7A02E0B7C6DDBA26C5FF33AEC" xlink:to="loc_country_US_9178C569EA8121380AB1A26C5FF310B8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GeographicDistributionForeignMember" xlink:label="loc_us-gaap_GeographicDistributionForeignMember_08C9553B97D6E48A775BA26C5FF3AC16" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4308E2D7A02E0B7C6DDBA26C5FF33AEC" xlink:to="loc_us-gaap_GeographicDistributionForeignMember_08C9553B97D6E48A775BA26C5FF3AC16" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_RestOfWorldMember" xlink:label="loc_biib_RestOfWorldMember_729C98B32B2F0BEBD6DCA26C5FF31A34" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4308E2D7A02E0B7C6DDBA26C5FF33AEC" xlink:to="loc_biib_RestOfWorldMember_729C98B32B2F0BEBD6DCA26C5FF31A34" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_A42F0112AF523E29FAF2A26C5FF40AF3" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:to="loc_us-gaap_StatementScenarioAxis_A42F0112AF523E29FAF2A26C5FF40AF3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_BE685AE066181F8E6422A26C5FF42C8F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_A42F0112AF523E29FAF2A26C5FF40AF3" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_BE685AE066181F8E6422A26C5FF42C8F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_BE685AE066181F8E6422A26C5FF42C8F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_A42F0112AF523E29FAF2A26C5FF40AF3" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_BE685AE066181F8E6422A26C5FF42C8F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioForecastMember" xlink:label="loc_us-gaap_ScenarioForecastMember_77BB31FDC2A770060855A26C5FF4CE43" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_BE685AE066181F8E6422A26C5FF42C8F" xlink:to="loc_us-gaap_ScenarioForecastMember_77BB31FDC2A770060855A26C5FF4CE43" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_1368CAF6E0B7171A244FA26C5FF9DE5B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:to="loc_us-gaap_TypeOfArrangementAxis_1368CAF6E0B7171A244FA26C5FF9DE5B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_1368CAF6E0B7171A244FA26C5FF9DE5B" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_1368CAF6E0B7171A244FA26C5FF9DE5B" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_IonisPharmaceuticalsMember" xlink:label="loc_biib_IonisPharmaceuticalsMember_B65A6CFDFD18C9F31F38A26C5FFA5E66" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA" xlink:to="loc_biib_IonisPharmaceuticalsMember_B65A6CFDFD18C9F31F38A26C5FFA5E66" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_AbbVieMember" xlink:label="loc_biib_AbbVieMember_3B16B7D25473AF54ABF5A26C5FFAAF92" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA" xlink:to="loc_biib_AbbVieMember_3B16B7D25473AF54ABF5A26C5FFAAF92" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CollaborativeArrangementMember" xlink:label="loc_us-gaap_CollaborativeArrangementMember_DF2F48C2CB8651847930A26C5FFB8B6F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA" xlink:to="loc_us-gaap_CollaborativeArrangementMember_DF2F48C2CB8651847930A26C5FFB8B6F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_EisaiMember" xlink:label="loc_biib_EisaiMember_126B5DD5F40EE5EDA0D9A26C5FFBA679" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA" xlink:to="loc_biib_EisaiMember_126B5DD5F40EE5EDA0D9A26C5FFBA679" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_RocheGroupGenentechMember" xlink:label="loc_biib_RocheGroupGenentechMember_D2920741A0C20320E1E9A26C5FFBBBBD" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_AF66D5D226145A40DAC6A26C5FFA83FA" xlink:to="loc_biib_RocheGroupGenentechMember_D2920741A0C20320E1E9A26C5FFBBBBD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_E0655D22378203280695A26C5FFBE37A" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_F6633FC7456C2F783F66A26C5FEF9E0E" xlink:to="loc_srt_ProductOrServiceAxis_E0655D22378203280695A26C5FFBE37A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_E0655D22378203280695A26C5FFBE37A" xlink:to="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_E0655D22378203280695A26C5FFBE37A" xlink:to="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ZINBRYTAMember" xlink:label="loc_biib_ZINBRYTAMember_0825BCDC0A0E6D747AF2A26C5FFC6146" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802" xlink:to="loc_biib_ZINBRYTAMember_0825BCDC0A0E6D747AF2A26C5FFC6146" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_E2609andBAN2401Member" xlink:label="loc_biib_E2609andBAN2401Member_F0C49B0D6F1F1C0649D6A26C5FFC87B1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802" xlink:to="loc_biib_E2609andBAN2401Member_F0C49B0D6F1F1C0649D6A26C5FFC87B1" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_AducanumabMember" xlink:label="loc_biib_AducanumabMember_C16BF03E745879E14B33A26C5FFC52B9" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802" xlink:to="loc_biib_AducanumabMember_C16BF03E745879E14B33A26C5FFC52B9" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_OCREVUSMember" xlink:label="loc_biib_OCREVUSMember_0D161DF697BB50E4A654A26C5FFC46D0" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802" xlink:to="loc_biib_OCREVUSMember_0D161DF697BB50E4A654A26C5FFC46D0" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Revenuesfromanticd20therapeuticprogramsMember" xlink:label="loc_biib_Revenuesfromanticd20therapeuticprogramsMember_8E8EB2FA17563EC59CF0A26C5FFCE559" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_1344D90D2633F8AD82A7A26C5FFB7802" xlink:to="loc_biib_Revenuesfromanticd20therapeuticprogramsMember_8E8EB2FA17563EC59CF0A26C5FFCE559" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Termofcollaborationagreement" xlink:label="loc_biib_Termofcollaborationagreement_6A60D01AD769D582F5CCA26C5FFDD620" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Termofcollaborationagreement_6A60D01AD769D582F5CCA26C5FFDD620" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Totalpaymenttoentercollaborationagreement" xlink:label="loc_biib_Totalpaymenttoentercollaborationagreement_804767424E8749836554A26C5FFDAC66" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Totalpaymenttoentercollaborationagreement_804767424E8749836554A26C5FFDAC66" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Upfrontpaymentforcollaborationagreement" xlink:label="loc_biib_Upfrontpaymentforcollaborationagreement_059471495BF92D76E5D6A26C5FFD11F8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Upfrontpaymentforcollaborationagreement_059471495BF92D76E5D6A26C5FFD11F8" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Investmentincommonstocksharespurchased" xlink:label="loc_biib_Investmentincommonstocksharespurchased_F4E41ED116BE420939C7A26C5FFD5D41" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Investmentincommonstocksharespurchased_F4E41ED116BE420939C7A26C5FFD5D41" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Purchaseofcommonstock" xlink:label="loc_biib_Purchaseofcommonstock_FC6018CF358B94C4E071A26C5FFE6D23" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Purchaseofcommonstock_FC6018CF358B94C4E071A26C5FFE6D23" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssets" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssets_5F5A898239A208E0961CA26C5FFE6996" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssets_5F5A898239A208E0961CA26C5FFE6996" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_1D088061496A77BCE77FA26C5FFE492F" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_1D088061496A77BCE77FA26C5FFE492F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Investmentincommonstockvalue" xlink:label="loc_biib_Investmentincommonstockvalue_19CD75550C716CAD633BA26C5FFE1D5D" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Investmentincommonstockvalue_19CD75550C716CAD633BA26C5FFE1D5D" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Premiumonpurchaseofcommonstock" xlink:label="loc_biib_Premiumonpurchaseofcommonstock_F2C4A6BFBD6F997B3C90A26C5FFE08FB" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Premiumonpurchaseofcommonstock_F2C4A6BFBD6F997B3C90A26C5FFE08FB" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones" xlink:label="loc_biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones_66E401F212C6823234CFA26C5FFEA01E" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones_66E401F212C6823234CFA26C5FFEA01E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CostOfGoodsAndServicesSold" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_2DCDA179A73C0EC26765A26C5FFF97B6" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_2DCDA179A73C0EC26765A26C5FFF97B6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_18DF212DFFE090386A2CA26C5FFF3FB5" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_us-gaap_Revenues_18DF212DFFE090386A2CA26C5FFF3FB5" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Collaborationprofitlosssharing" xlink:label="loc_biib_Collaborationprofitlosssharing_FE755F1DEA6CA31BFFB3A26C5FFF9D74" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Collaborationprofitlosssharing_FE755F1DEA6CA31BFFB3A26C5FFF9D74" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Biogenshareofcopromotionprofitsorlosses" xlink:label="loc_biib_Biogenshareofcopromotionprofitsorlosses_54A5549C137E525135B9A26C5FFF15D1" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Biogenshareofcopromotionprofitsorlosses_54A5549C137E525135B9A26C5FFF15D1" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Expenseincurredbythecollaboration" xlink:label="loc_biib_Expenseincurredbythecollaboration_4F8610DF48B54E178AF1A26C5FFF1F51" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Expenseincurredbythecollaboration_4F8610DF48B54E178AF1A26C5FFF1F51" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Expensereflectedwithinstatementsofincome" xlink:label="loc_biib_Expensereflectedwithinstatementsofincome_EFEB9F9EF6FB7B9F8AF7A26C5FFF357E" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Expensereflectedwithinstatementsofincome_EFEB9F9EF6FB7B9F8AF7A26C5FFF357E" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PercentageoffuturedevelopmentcostsrelatedtoEisai" xlink:label="loc_biib_PercentageoffuturedevelopmentcostsrelatedtoEisai_52F1F68E6AD44A3C1C48A26C60007E3F" xlink:type="locator" />
    <link:definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_PercentageoffuturedevelopmentcostsrelatedtoEisai_52F1F68E6AD44A3C1C48A26C60007E3F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_RoyaltiesPercentageToBeReceivedOnSaleOfProduct" xlink:label="loc_biib_RoyaltiesPercentageToBeReceivedOnSaleOfProduct_E19384FF76515B69D81DA26C6000A96B" xlink:type="locator" />
    <link:definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_RoyaltiesPercentageToBeReceivedOnSaleOfProduct_E19384FF76515B69D81DA26C6000A96B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Royaltyoperatingprofitthresholdforhighestroyaltyratepercentage" xlink:label="loc_biib_Royaltyoperatingprofitthresholdforhighestroyaltyratepercentage_7F4B900E59D2EE02172BA26C600523D7" xlink:type="locator" />
    <link:definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Royaltyoperatingprofitthresholdforhighestroyaltyratepercentage_7F4B900E59D2EE02172BA26C600523D7" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Reductioninroyaltyrate" xlink:label="loc_biib_Reductioninroyaltyrate_A8FE687641292E4D1D87A26C60057868" xlink:type="locator" />
    <link:definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_Reductioninroyaltyrate_A8FE687641292E4D1D87A26C60057868" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PeriodOfCollaborationAgreement" xlink:label="loc_biib_PeriodOfCollaborationAgreement_E9AF54B8334FA04F8D2BA26C6006FF61" xlink:type="locator" />
    <link:definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_PeriodOfCollaborationAgreement_E9AF54B8334FA04F8D2BA26C6006FF61" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ShareOfCoPromotionProfits" xlink:label="loc_biib_ShareOfCoPromotionProfits_45CFDB4B26620A8B23E9A26C600621F6" xlink:type="locator" />
    <link:definitionArc order="22" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_ShareOfCoPromotionProfits_45CFDB4B26620A8B23E9A26C600621F6" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_OtherrevenuesfromantiCD20therapeuticprograms" xlink:label="loc_biib_OtherrevenuesfromantiCD20therapeuticprograms_749E3596EA131C70B8C9A26C600601EB" xlink:type="locator" />
    <link:definitionArc order="23" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_F42E401E665C6A3F9CFCA26C5FFDD348" xlink:to="loc_biib_OtherrevenuesfromantiCD20therapeuticprograms_749E3596EA131C70B8C9A26C600601EB" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsEquityMethodInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_48F45529630E6EA03172A26C5FCB9EE7" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_2B0C7F7212B77ED1B5E1A26C5FC981CF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_48F45529630E6EA03172A26C5FCB9EE7" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_2B0C7F7212B77ED1B5E1A26C5FC981CF" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_7F1B7BE0D0DB04D19F77A26C5FCAA8E5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_2B0C7F7212B77ED1B5E1A26C5FC981CF" xlink:to="loc_us-gaap_TypeOfArrangementAxis_7F1B7BE0D0DB04D19F77A26C5FCAA8E5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2BC64486B2BED1942D7DA26C5FCA545F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_7F1B7BE0D0DB04D19F77A26C5FCAA8E5" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2BC64486B2BED1942D7DA26C5FCA545F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2BC64486B2BED1942D7DA26C5FCA545F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_7F1B7BE0D0DB04D19F77A26C5FCAA8E5" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2BC64486B2BED1942D7DA26C5FCA545F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CollaborativeArrangementMember" xlink:label="loc_us-gaap_CollaborativeArrangementMember_8DF358F1DA24BD723546A26C5FCA720E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2BC64486B2BED1942D7DA26C5FCA545F" xlink:to="loc_us-gaap_CollaborativeArrangementMember_8DF358F1DA24BD723546A26C5FCA720E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_90BDA49BEBBDC13E4FD7A26C5FCA742A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_2B0C7F7212B77ED1B5E1A26C5FC981CF" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_90BDA49BEBBDC13E4FD7A26C5FCA742A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_FBCCCAB3D5E60D77CDE1A26C5FCB2685_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_90BDA49BEBBDC13E4FD7A26C5FCA742A" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_FBCCCAB3D5E60D77CDE1A26C5FCB2685_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_FBCCCAB3D5E60D77CDE1A26C5FCB2685" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_90BDA49BEBBDC13E4FD7A26C5FCA742A" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_FBCCCAB3D5E60D77CDE1A26C5FCB2685" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SamsungBiosimilarAgreementMember" xlink:label="loc_biib_SamsungBiosimilarAgreementMember_C92F236253D3AAF5BAB4A26C5FCB280C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_FBCCCAB3D5E60D77CDE1A26C5FCB2685" xlink:to="loc_biib_SamsungBiosimilarAgreementMember_C92F236253D3AAF5BAB4A26C5FCB280C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_F4BCEFE736A07F34A861A26C5FCB1B92" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_48F45529630E6EA03172A26C5FCB9EE7" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_F4BCEFE736A07F34A861A26C5FCB1B92" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_EquityMethodInvestmentOwnershipPercentageMaximum" xlink:label="loc_biib_EquityMethodInvestmentOwnershipPercentageMaximum_AB3D4D392DB95267C022A26C5FCC375C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_48F45529630E6EA03172A26C5FCB9EE7" xlink:to="loc_biib_EquityMethodInvestmentOwnershipPercentageMaximum_AB3D4D392DB95267C022A26C5FCC375C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments" xlink:label="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_B133EB70A3A99A2BFFE2A26C5FCCC818" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_48F45529630E6EA03172A26C5FCB9EE7" xlink:to="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_B133EB70A3A99A2BFFE2A26C5FCCC818" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Biogenshareofcopromotionprofitsorlosses" xlink:label="loc_biib_Biogenshareofcopromotionprofitsorlosses_D7076815D3182F653554A26C5FCC98C3" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_48F45529630E6EA03172A26C5FCB9EE7" xlink:to="loc_biib_Biogenshareofcopromotionprofitsorlosses_D7076815D3182F653554A26C5FCC98C3" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Collaborationprofitlosssharing" xlink:label="loc_biib_Collaborationprofitlosssharing_F6F94B50F601C49B1367A26C5FCC7CDB" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_48F45529630E6EA03172A26C5FCB9EE7" xlink:to="loc_biib_Collaborationprofitlosssharing_F6F94B50F601C49B1367A26C5FCC7CDB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_FFF6A8F8365770A0FC3EA26C5FCCF9EB" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_48F45529630E6EA03172A26C5FCB9EE7" xlink:to="loc_us-gaap_Revenues_FFF6A8F8365770A0FC3EA26C5FCCF9EB" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CondensedConsolidatedBalanceSheetsUnaudited" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CondensedConsolidatedBalanceSheetsUnauditedParenthetical" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfComprehensiveIncomeUnaudited" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfIncomeUnaudited" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_0DC686E6843094E5958F9877D354C42A" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_1CE49C5BCCCD1EE9FCAA9877D350E565" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_0DC686E6843094E5958F9877D354C42A" xlink:to="loc_us-gaap_StatementTable_1CE49C5BCCCD1EE9FCAA9877D350E565" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_C72345D6D32744F1A0EC9877D350B08A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_1CE49C5BCCCD1EE9FCAA9877D350E565" xlink:to="loc_srt_ProductOrServiceAxis_C72345D6D32744F1A0EC9877D350B08A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_C3C333B621683CA39B1E9877D351C3A7_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_C72345D6D32744F1A0EC9877D350B08A" xlink:to="loc_srt_ProductsAndServicesDomain_C3C333B621683CA39B1E9877D351C3A7_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_C3C333B621683CA39B1E9877D351C3A7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_C72345D6D32744F1A0EC9877D350B08A" xlink:to="loc_srt_ProductsAndServicesDomain_C3C333B621683CA39B1E9877D351C3A7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProductMember" xlink:label="loc_us-gaap_ProductMember_EDB97835B4C5BFB0876C9877D35115EE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_C3C333B621683CA39B1E9877D351C3A7" xlink:to="loc_us-gaap_ProductMember_EDB97835B4C5BFB0876C9877D35115EE" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Revenuesfromanticd20therapeuticprogramsMember" xlink:label="loc_biib_Revenuesfromanticd20therapeuticprogramsMember_AD87F672CCFC458A8A3F9877D353924C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_C3C333B621683CA39B1E9877D351C3A7" xlink:to="loc_biib_Revenuesfromanticd20therapeuticprogramsMember_AD87F672CCFC458A8A3F9877D353924C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProductAndServiceOtherMember" xlink:label="loc_us-gaap_ProductAndServiceOtherMember_EBFB9270718993D260DD9877D3540B0D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_C3C333B621683CA39B1E9877D351C3A7" xlink:to="loc_us-gaap_ProductAndServiceOtherMember_EBFB9270718993D260DD9877D3540B0D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/DerivativeInstruments" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/DerivativeInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativesFairValueLineItems" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_6F4B3BDF4DBD5A728B1082090488D5EB" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_6F4B3BDF4DBD5A728B1082090488D5EB" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DerivativeMaturityAxis" xlink:label="loc_biib_DerivativeMaturityAxis_74DA8046E886B69130798209047AF239" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_biib_DerivativeMaturityAxis_74DA8046E886B69130798209047AF239" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DerivativeMaturityDomain" xlink:label="loc_biib_DerivativeMaturityDomain_0330099CDABD6ED3D0A88209047ACB76_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_DerivativeMaturityAxis_74DA8046E886B69130798209047AF239" xlink:to="loc_biib_DerivativeMaturityDomain_0330099CDABD6ED3D0A88209047ACB76_default" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DerivativeMaturityDomain" xlink:label="loc_biib_DerivativeMaturityDomain_0330099CDABD6ED3D0A88209047ACB76" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_DerivativeMaturityAxis_74DA8046E886B69130798209047AF239" xlink:to="loc_biib_DerivativeMaturityDomain_0330099CDABD6ED3D0A88209047ACB76" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ShorttermderivativeMember" xlink:label="loc_biib_ShorttermderivativeMember_C7C3D98CBE40BD01EF608209047A086C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeMaturityDomain_0330099CDABD6ED3D0A88209047ACB76" xlink:to="loc_biib_ShorttermderivativeMember_C7C3D98CBE40BD01EF608209047A086C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_FF6B592E0AB03C4B75F28209047B9A2D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_us-gaap_DebtInstrumentAxis_FF6B592E0AB03C4B75F28209047B9A2D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_3793041704531FAB8D9E8209047B4D7D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_FF6B592E0AB03C4B75F28209047B9A2D" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_3793041704531FAB8D9E8209047B4D7D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_3793041704531FAB8D9E8209047B4D7D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_FF6B592E0AB03C4B75F28209047B9A2D" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_3793041704531FAB8D9E8209047B4D7D" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember" xlink:label="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_52DA4C73FA1D6A44B2538209047BA986" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_3793041704531FAB8D9E8209047B4D7D" xlink:to="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_52DA4C73FA1D6A44B2538209047BA986" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_CurrencyAxis" xlink:label="loc_srt_CurrencyAxis_1D36CA9F3AD00E261BAF8209047C37F6" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_srt_CurrencyAxis_1D36CA9F3AD00E261BAF8209047C37F6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_AllCurrenciesDomain" xlink:label="loc_currency_AllCurrenciesDomain_04758AC9F0CC661362068209047CD833_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CurrencyAxis_1D36CA9F3AD00E261BAF8209047C37F6" xlink:to="loc_currency_AllCurrenciesDomain_04758AC9F0CC661362068209047CD833_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_AllCurrenciesDomain" xlink:label="loc_currency_AllCurrenciesDomain_04758AC9F0CC661362068209047CD833" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CurrencyAxis_1D36CA9F3AD00E261BAF8209047C37F6" xlink:to="loc_currency_AllCurrenciesDomain_04758AC9F0CC661362068209047CD833" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_EUR" xlink:label="loc_currency_EUR_62B376B359780894A9A88209047C7FD7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_04758AC9F0CC661362068209047CD833" xlink:to="loc_currency_EUR_62B376B359780894A9A88209047C7FD7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_GBP" xlink:label="loc_currency_GBP_79720159C72BCEB7CF978209047C6AB6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_04758AC9F0CC661362068209047CD833" xlink:to="loc_currency_GBP_79720159C72BCEB7CF978209047C6AB6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_CAD" xlink:label="loc_currency_CAD_30A273170BB5880369C9820904828BEC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_04758AC9F0CC661362068209047CD833" xlink:to="loc_currency_CAD_30A273170BB5880369C9820904828BEC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_CHF" xlink:label="loc_currency_CHF_33F6B0A8D4984684EEFB82090482F05A" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_04758AC9F0CC661362068209047CD833" xlink:to="loc_currency_CHF_33F6B0A8D4984684EEFB82090482F05A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_F794B7EA0545B579BFDE820904829F95" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_F794B7EA0545B579BFDE820904829F95" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_1478815B1C14E367F61482090482330C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_F794B7EA0545B579BFDE820904829F95" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_1478815B1C14E367F61482090482330C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_1478815B1C14E367F61482090482330C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_F794B7EA0545B579BFDE820904829F95" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_1478815B1C14E367F61482090482330C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SalesMember" xlink:label="loc_us-gaap_SalesMember_3F18CC7C11EEAE16887A82090482FB12" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1478815B1C14E367F61482090482330C" xlink:to="loc_us-gaap_SalesMember_3F18CC7C11EEAE16887A82090482FB12" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingExpenseMember" xlink:label="loc_us-gaap_OperatingExpenseMember_B2CB2CDF92057D8E253A82090483F797" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1478815B1C14E367F61482090482330C" xlink:to="loc_us-gaap_OperatingExpenseMember_B2CB2CDF92057D8E253A82090483F797" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpenseMember" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpenseMember_1ABBB377DBB112125F9982090483B9E1" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1478815B1C14E367F61482090482330C" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpenseMember_1ABBB377DBB112125F9982090483B9E1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_4AA82FC43E2860AC61238209048361CE" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_4AA82FC43E2860AC61238209048361CE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="loc_us-gaap_HedgingRelationshipDomain_1668A28F72ADEEED2DCE82090483EB4B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_4AA82FC43E2860AC61238209048361CE" xlink:to="loc_us-gaap_HedgingRelationshipDomain_1668A28F72ADEEED2DCE82090483EB4B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="loc_us-gaap_HedgingRelationshipDomain_1668A28F72ADEEED2DCE82090483EB4B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_4AA82FC43E2860AC61238209048361CE" xlink:to="loc_us-gaap_HedgingRelationshipDomain_1668A28F72ADEEED2DCE82090483EB4B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_CashflowsrevenueMember" xlink:label="loc_biib_CashflowsrevenueMember_5DC47E19D336739F61ED82090484C81D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_1668A28F72ADEEED2DCE82090483EB4B" xlink:to="loc_biib_CashflowsrevenueMember_5DC47E19D336739F61ED82090484C81D" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_CashflowsoperatingexpensesMember" xlink:label="loc_biib_CashflowsoperatingexpensesMember_B351F892942EB3E47A85820904849448" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_1668A28F72ADEEED2DCE82090483EB4B" xlink:to="loc_biib_CashflowsoperatingexpensesMember_B351F892942EB3E47A85820904849448" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_1ACAE34F0BB08E1FE5398209048477E1" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_1ACAE34F0BB08E1FE5398209048477E1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_CD0A0F5D416B56B91E7082090484054E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_1ACAE34F0BB08E1FE5398209048477E1" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_CD0A0F5D416B56B91E7082090484054E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_CD0A0F5D416B56B91E7082090484054E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_1ACAE34F0BB08E1FE5398209048477E1" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_CD0A0F5D416B56B91E7082090484054E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ForeignExchangeContractMember" xlink:label="loc_us-gaap_ForeignExchangeContractMember_773EBDB04261BF13962582090484EDDC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_CD0A0F5D416B56B91E7082090484054E" xlink:to="loc_us-gaap_ForeignExchangeContractMember_773EBDB04261BF13962582090484EDDC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InterestRateSwapMember" xlink:label="loc_us-gaap_InterestRateSwapMember_D375A2739EDF42DCC46E82090485FEEC" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_CD0A0F5D416B56B91E7082090484054E" xlink:to="loc_us-gaap_InterestRateSwapMember_D375A2739EDF42DCC46E82090485FEEC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_37F1A98CE4F563C0F30D820904855CC7" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_37F1A98CE4F563C0F30D820904855CC7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_041BD7AB82FBAFC4AAAA820904850C3A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_37F1A98CE4F563C0F30D820904855CC7" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_041BD7AB82FBAFC4AAAA820904850C3A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_041BD7AB82FBAFC4AAAA820904850C3A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_37F1A98CE4F563C0F30D820904855CC7" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_041BD7AB82FBAFC4AAAA820904850C3A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCurrentAssetsMember" xlink:label="loc_us-gaap_OtherCurrentAssetsMember_903F4B8F97CE72B8730782090485B1EF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_041BD7AB82FBAFC4AAAA820904850C3A" xlink:to="loc_us-gaap_OtherCurrentAssetsMember_903F4B8F97CE72B8730782090485B1EF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherInvestmentsMember" xlink:label="loc_us-gaap_OtherInvestmentsMember_8EB619066F947750578E826C87AE013E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_041BD7AB82FBAFC4AAAA820904850C3A" xlink:to="loc_us-gaap_OtherInvestmentsMember_8EB619066F947750578E826C87AE013E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccruedLiabilitiesMember" xlink:label="loc_us-gaap_AccruedLiabilitiesMember_A18759B3E3688C1A6E0F820904861733" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_041BD7AB82FBAFC4AAAA820904850C3A" xlink:to="loc_us-gaap_AccruedLiabilitiesMember_A18759B3E3688C1A6E0F820904861733" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherNoncurrentLiabilitiesMember" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_71C8E800E7413CD1BC30820904864C80" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_041BD7AB82FBAFC4AAAA820904850C3A" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_71C8E800E7413CD1BC30820904864C80" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="loc_us-gaap_HedgingDesignationAxis_46A9D1E484732C335102820904864564" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_us-gaap_HedgingDesignationAxis_46A9D1E484732C335102820904864564" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_0C1EFFADE1F79E10E3ED8209048613FD_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_46A9D1E484732C335102820904864564" xlink:to="loc_us-gaap_HedgingDesignationDomain_0C1EFFADE1F79E10E3ED8209048613FD_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_0C1EFFADE1F79E10E3ED8209048613FD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_46A9D1E484732C335102820904864564" xlink:to="loc_us-gaap_HedgingDesignationDomain_0C1EFFADE1F79E10E3ED8209048613FD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_D60AE2F2FD2F9BF76C408209048762FB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_0C1EFFADE1F79E10E3ED8209048613FD" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_D60AE2F2FD2F9BF76C408209048762FB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="loc_us-gaap_NondesignatedMember_7879C3DDB33FD9D11615820904875A8B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_0C1EFFADE1F79E10E3ED8209048613FD" xlink:to="loc_us-gaap_NondesignatedMember_7879C3DDB33FD9D11615820904875A8B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_D6A6AB1F1E1C6F98D00882090487A2D9" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_1B17B954719EB1354436820904796899" xlink:to="loc_srt_RangeAxis_D6A6AB1F1E1C6F98D00882090487A2D9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_CE10D2447B1935BE9BAD82090487C14D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_D6A6AB1F1E1C6F98D00882090487A2D9" xlink:to="loc_srt_RangeMember_CE10D2447B1935BE9BAD82090487C14D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_CE10D2447B1935BE9BAD82090487C14D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_D6A6AB1F1E1C6F98D00882090487A2D9" xlink:to="loc_srt_RangeMember_CE10D2447B1935BE9BAD82090487C14D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_315420EDA1BB345135C982090487EC2A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_CE10D2447B1935BE9BAD82090487C14D" xlink:to="loc_srt_MinimumMember_315420EDA1BB345135C982090487EC2A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_09FFCF2EBCE952F5036B820904881998" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_CE10D2447B1935BE9BAD82090487C14D" xlink:to="loc_srt_MaximumMember_09FFCF2EBCE952F5036B820904881998" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeAssetFairValueNetAbstract" xlink:label="loc_us-gaap_DerivativeAssetFairValueNetAbstract_71B155E738C9F31514E8820904883A91" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_6F4B3BDF4DBD5A728B1082090488D5EB" xlink:to="loc_us-gaap_DerivativeAssetFairValueNetAbstract_71B155E738C9F31514E8820904883A91" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_04228DAD98E6D4B5D4D0820904886891" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetFairValueNetAbstract_71B155E738C9F31514E8820904883A91" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_04228DAD98E6D4B5D4D0820904886891" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_32938F69AB64D56762BE82090489DA90" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetFairValueNetAbstract_71B155E738C9F31514E8820904883A91" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_32938F69AB64D56762BE82090489DA90" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DerivativeInstrumentsTextualAbstract" xlink:label="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_6F4B3BDF4DBD5A728B1082090488D5EB" xlink:to="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeRemainingMaturity1" xlink:label="loc_us-gaap_DerivativeRemainingMaturity1_EF307BE15829ED4DE76282090489E387" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:to="loc_us-gaap_DerivativeRemainingMaturity1_EF307BE15829ED4DE76282090489E387" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd#invest_DerivativeNotionalAmount" xlink:label="loc_invest_DerivativeNotionalAmount_A03793E29C1D9D4DD2308209048E4522" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:to="loc_invest_DerivativeNotionalAmount_A03793E29C1D9D4DD2308209048E4522" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_UnrealizedGainLossesOnForeignCurrencyDerivativesNetBeforeTax" xlink:label="loc_biib_UnrealizedGainLossesOnForeignCurrencyDerivativesNetBeforeTax_1C20B126B1B6123A79028209048E206A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:to="loc_biib_UnrealizedGainLossesOnForeignCurrencyDerivativesNetBeforeTax_1C20B126B1B6123A79028209048E206A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_UnrealizedGainLossOnForeignCurrencyDerivativesNetBeforeTax" xlink:label="loc_us-gaap_UnrealizedGainLossOnForeignCurrencyDerivativesNetBeforeTax_51978725DBBBA5EFE8AA8209048FEEA5" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:to="loc_us-gaap_UnrealizedGainLossOnForeignCurrencyDerivativesNetBeforeTax_51978725DBBBA5EFE8AA8209048FEEA5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet_4978198B2EE81A64139E8209048FEA87" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet_4978198B2EE81A64139E8209048FEA87" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet_80874CC80D06D6E677898209048FE5E7" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet_80874CC80D06D6E677898209048FE5E7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_E3D3F9CCBA8C17927B2A8209048F67AC" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_E3D3F9CCBA8C17927B2A8209048F67AC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments" xlink:label="loc_us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments_8EAA8DE41FE25C57B7528209048FE9D4" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DerivativeInstrumentsTextualAbstract_98BEF039FBC4F8C3F1E88209048923AE" xlink:to="loc_us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments_8EAA8DE41FE25C57B7528209048FE9D4" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/DerivativeInstrumentsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/DocumentAndEntityInformation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/EarningsPerShare" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/EarningsPerShareDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8335ADD15CFF54A585705E337DF1C58C" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_B8298F7AE7885C179F4C0412DA77AB36" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8335ADD15CFF54A585705E337DF1C58C" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_B8298F7AE7885C179F4C0412DA77AB36" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_47CDDC8A1E025CA59A7D66D13D31BF11" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_B8298F7AE7885C179F4C0412DA77AB36" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_47CDDC8A1E025CA59A7D66D13D31BF11" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_F340E3A1EA3A6A18912D3C312D43A386_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_47CDDC8A1E025CA59A7D66D13D31BF11" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_F340E3A1EA3A6A18912D3C312D43A386_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_5C3A9B9569E75001A313B0AEE2CA8773" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_47CDDC8A1E025CA59A7D66D13D31BF11" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_5C3A9B9569E75001A313B0AEE2CA8773" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_StockOptionsAndEmployeeStockPurchasePlanMember" xlink:label="loc_biib_StockOptionsAndEmployeeStockPurchasePlanMember_430EFF93B60D5A1F85F8354950E82F0E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_5C3A9B9569E75001A313B0AEE2CA8773" xlink:to="loc_biib_StockOptionsAndEmployeeStockPurchasePlanMember_430EFF93B60D5A1F85F8354950E82F0E" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TimeVestedRestrictedStockUnitsMember" xlink:label="loc_biib_TimeVestedRestrictedStockUnitsMember_CE0C32B91F1853ACA392E85CED606C62" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_5C3A9B9569E75001A313B0AEE2CA8773" xlink:to="loc_biib_TimeVestedRestrictedStockUnitsMember_CE0C32B91F1853ACA392E85CED606C62" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_MarketStockUnitsMember" xlink:label="loc_biib_MarketStockUnitsMember_EDC9DFCB14EF594ABE565AFE5A530795" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_5C3A9B9569E75001A313B0AEE2CA8773" xlink:to="loc_biib_MarketStockUnitsMember_EDC9DFCB14EF594ABE565AFE5A530795" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PerformanceStockUnitsSettledinStockMember" xlink:label="loc_biib_PerformanceStockUnitsSettledinStockMember_B590FD5F6D855BCC8E55F530CAE5C99B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_5C3A9B9569E75001A313B0AEE2CA8773" xlink:to="loc_biib_PerformanceStockUnitsSettledinStockMember_B590FD5F6D855BCC8E55F530CAE5C99B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_BasicAndDilutedEarningsPerShareAbstract" xlink:label="loc_biib_BasicAndDilutedEarningsPerShareAbstract_47DFBBD7B3FB545CBD084824781C2DA3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8335ADD15CFF54A585705E337DF1C58C" xlink:to="loc_biib_BasicAndDilutedEarningsPerShareAbstract_47DFBBD7B3FB545CBD084824781C2DA3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLossAbstract" xlink:label="loc_us-gaap_NetIncomeLossAbstract_F1C613149E535865B599D39F2B56E2E3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_BasicAndDilutedEarningsPerShareAbstract_47DFBBD7B3FB545CBD084824781C2DA3" xlink:to="loc_us-gaap_NetIncomeLossAbstract_F1C613149E535865B599D39F2B56E2E3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_B3FA82A7F2BF52F6B3E259F61BF05699" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAbstract_F1C613149E535865B599D39F2B56E2E3" xlink:to="loc_us-gaap_NetIncomeLoss_B3FA82A7F2BF52F6B3E259F61BF05699" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E6747F6AABE15C7B95F509D2FC1142A3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_BasicAndDilutedEarningsPerShareAbstract_47DFBBD7B3FB545CBD084824781C2DA3" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E6747F6AABE15C7B95F509D2FC1142A3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_4E4D629915DC5D9B9E8875C5C8C533C0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E6747F6AABE15C7B95F509D2FC1142A3" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_4E4D629915DC5D9B9E8875C5C8C533C0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DilutiveSecuritiesAbstract" xlink:label="loc_us-gaap_DilutiveSecuritiesAbstract_CC937EBBE38256339DAEB03C979F6EA5" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E6747F6AABE15C7B95F509D2FC1142A3" xlink:to="loc_us-gaap_DilutiveSecuritiesAbstract_CC937EBBE38256339DAEB03C979F6EA5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_F239C962FB885349A706DA10773D7FC4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DilutiveSecuritiesAbstract_CC937EBBE38256339DAEB03C979F6EA5" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_F239C962FB885349A706DA10773D7FC4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_0A3AD2ADB59656D28F4C555EBCA0A983" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DilutiveSecuritiesAbstract_CC937EBBE38256339DAEB03C979F6EA5" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_0A3AD2ADB59656D28F4C555EBCA0A983" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_120CF56D2CC95ACD92FEFA08D1449E8A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E6747F6AABE15C7B95F509D2FC1142A3" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_120CF56D2CC95ACD92FEFA08D1449E8A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/EarningsPerShareTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/Equity" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/EquityDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/EquityDetailsTextual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_3E8B8D93BC609DD9E5D01739FEB534CD" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_DD78CF7B15E005DEA8B11739FEAD8A04" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E8B8D93BC609DD9E5D01739FEB534CD" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_DD78CF7B15E005DEA8B11739FEAD8A04" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_FD65DF84834E3DB3BD531739FEAD067F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_DD78CF7B15E005DEA8B11739FEAD8A04" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_FD65DF84834E3DB3BD531739FEAD067F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_90CFCF9F49DA5868E7131739FEAEAF6B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_FD65DF84834E3DB3BD531739FEAD067F" xlink:to="loc_us-gaap_TypeOfAdoptionMember_90CFCF9F49DA5868E7131739FEAEAF6B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_90CFCF9F49DA5868E7131739FEAEAF6B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_FD65DF84834E3DB3BD531739FEAD067F" xlink:to="loc_us-gaap_TypeOfAdoptionMember_90CFCF9F49DA5868E7131739FEAEAF6B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountingStandardsUpdate201616Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201616Member_3B047112BB3522DEE4201739FEAEB042" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_90CFCF9F49DA5868E7131739FEAEAF6B" xlink:to="loc_us-gaap_AccountingStandardsUpdate201616Member_3B047112BB3522DEE4201739FEAEB042" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_54B0BFA0D3CBB0B06CB71739FEAE30A1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_DD78CF7B15E005DEA8B11739FEAD8A04" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_54B0BFA0D3CBB0B06CB71739FEAE30A1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_3841DAA1F265B56951DA1739FEAFC3DD_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_54B0BFA0D3CBB0B06CB71739FEAE30A1" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_3841DAA1F265B56951DA1739FEAFC3DD_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_3841DAA1F265B56951DA1739FEAFC3DD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_54B0BFA0D3CBB0B06CB71739FEAE30A1" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_3841DAA1F265B56951DA1739FEAFC3DD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_6638AD4AD92A1C5DC8A21739FEAFDE08" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_3841DAA1F265B56951DA1739FEAFC3DD" xlink:to="loc_us-gaap_RetainedEarningsMember_6638AD4AD92A1C5DC8A21739FEAFDE08" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_299B25422AF6C28FBE571739FEAF7281" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_DD78CF7B15E005DEA8B11739FEAD8A04" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_299B25422AF6C28FBE571739FEAF7281" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_2F3E0F4AD997FF0552801739FEB40C9A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_299B25422AF6C28FBE571739FEAF7281" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_2F3E0F4AD997FF0552801739FEB40C9A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_2F3E0F4AD997FF0552801739FEB40C9A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_299B25422AF6C28FBE571739FEAF7281" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_2F3E0F4AD997FF0552801739FEB40C9A" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_A2018ShareRepurchaseProgramMember" xlink:label="loc_biib_A2018ShareRepurchaseProgramMember_628A7F600588C8703D1A173A11BC4AF6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_2F3E0F4AD997FF0552801739FEB40C9A" xlink:to="loc_biib_A2018ShareRepurchaseProgramMember_628A7F600588C8703D1A173A11BC4AF6" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_A2016ShareRepurchaseProgramMember" xlink:label="loc_biib_A2016ShareRepurchaseProgramMember_EF293E7AC27A2AC6E8121739FEB59A20" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_2F3E0F4AD997FF0552801739FEB40C9A" xlink:to="loc_biib_A2016ShareRepurchaseProgramMember_EF293E7AC27A2AC6E8121739FEB59A20" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_A2011ShareRepurchaseProgramMember" xlink:label="loc_biib_A2011ShareRepurchaseProgramMember_52F5741D26E28F1F48A81739FEB51101" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_2F3E0F4AD997FF0552801739FEB40C9A" xlink:to="loc_biib_A2011ShareRepurchaseProgramMember_52F5741D26E28F1F48A81739FEB51101" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1" xlink:label="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_D407FD40186820AB289B1739FEB5A1D4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E8B8D93BC609DD9E5D01739FEB534CD" xlink:to="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_D407FD40186820AB289B1739FEB5A1D4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" xlink:label="loc_us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased_3D79FE9EC34E17FD4E3C1739FEB6CB4A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E8B8D93BC609DD9E5D01739FEB534CD" xlink:to="loc_us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased_3D79FE9EC34E17FD4E3C1739FEB6CB4A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_28536CC613047AE673D81739FEB651D2" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E8B8D93BC609DD9E5D01739FEB534CD" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_28536CC613047AE673D81739FEB651D2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TreasuryStockSharesAcquired" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_C6B1905552249EC075931739FEB65BD1" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E8B8D93BC609DD9E5D01739FEB534CD" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_C6B1905552249EC075931739FEB65BD1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:label="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_75261F13141FAD2D57D11739FEB6EBFF" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_3E8B8D93BC609DD9E5D01739FEB534CD" xlink:to="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_75261F13141FAD2D57D11739FEB6EBFF" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/EquityTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurements" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_C6DAA2BE0B670F6772D97D463478E783" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_40C5BD9256369B289CF77D463475EB55" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_C6DAA2BE0B670F6772D97D463478E783" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_40C5BD9256369B289CF77D463475EB55" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_93F409EF6E0357AABBD27D463475E109" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_40C5BD9256369B289CF77D463475EB55" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_93F409EF6E0357AABBD27D463475E109" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0DDF6F5FC24F03999D0A7D463475937D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_93F409EF6E0357AABBD27D463475E109" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0DDF6F5FC24F03999D0A7D463475937D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0DDF6F5FC24F03999D0A7D463475937D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_93F409EF6E0357AABBD27D463475E109" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0DDF6F5FC24F03999D0A7D463475937D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_781CC853D9D2692FA6917D46347641C1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0DDF6F5FC24F03999D0A7D463475937D" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_781CC853D9D2692FA6917D46347641C1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_88128782F588D0B438D17D463476BC23" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0DDF6F5FC24F03999D0A7D463475937D" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_88128782F588D0B438D17D463476BC23" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_3427195E7D99F724C8637D4634767BD8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0DDF6F5FC24F03999D0A7D463475937D" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_3427195E7D99F724C8637D4634767BD8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_EBAEA65544054E908EEE7D4634765F4E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_40C5BD9256369B289CF77D463475EB55" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_EBAEA65544054E908EEE7D4634765F4E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_DD4ADCDCB8FD1FD4F8237D4634768FD5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_EBAEA65544054E908EEE7D4634765F4E" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_DD4ADCDCB8FD1FD4F8237D4634768FD5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_DD4ADCDCB8FD1FD4F8237D4634768FD5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_EBAEA65544054E908EEE7D4634765F4E" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_DD4ADCDCB8FD1FD4F8237D4634768FD5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_35F503D6191CAABBA1E37D463477DCA2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_DD4ADCDCB8FD1FD4F8237D4634768FD5" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_35F503D6191CAABBA1E37D463477DCA2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_70BAA40683BEEB7E86E87D46347794BC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_40C5BD9256369B289CF77D463475EB55" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_70BAA40683BEEB7E86E87D46347794BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9640ABE16004E466452C7D4634772358_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_70BAA40683BEEB7E86E87D46347794BC" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9640ABE16004E466452C7D4634772358_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9640ABE16004E466452C7D4634772358" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_70BAA40683BEEB7E86E87D46347794BC" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9640ABE16004E466452C7D4634772358" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_9E111C53A07AA43D0B067D4634772F0B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9640ABE16004E466452C7D4634772358" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_9E111C53A07AA43D0B067D4634772F0B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_7F7AC4BCFF4CCB8E22E57D4634780DD3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9640ABE16004E466452C7D4634772358" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_7F7AC4BCFF4CCB8E22E57D4634780DD3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetBackedSecuritiesMember" xlink:label="loc_us-gaap_AssetBackedSecuritiesMember_F95566211C12C030CD5A7D463478E33A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_9640ABE16004E466452C7D4634772358" xlink:to="loc_us-gaap_AssetBackedSecuritiesMember_F95566211C12C030CD5A7D463478E33A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisAbstract" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisAbstract_FED8DAF3C7A68E56FCD37D4634782106" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_C6DAA2BE0B670F6772D97D463478E783" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisAbstract_FED8DAF3C7A68E56FCD37D4634782106" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsFairValueDisclosureAbstract" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_B73284B9B4C48ABA217D7D463478E539" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisAbstract_FED8DAF3C7A68E56FCD37D4634782106" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_B73284B9B4C48ABA217D7D463478E539" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_F30181A620C5832CC1DF7D4634794772" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_B73284B9B4C48ABA217D7D463478E539" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_F30181A620C5832CC1DF7D4634794772" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_05713B95151892819B487D46347992DD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_B73284B9B4C48ABA217D7D463478E539" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_05713B95151892819B487D46347992DD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentsFairValueDisclosure" xlink:label="loc_us-gaap_InvestmentsFairValueDisclosure_D47247AB295A42CFB70E7D4634796561" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_B73284B9B4C48ABA217D7D463478E539" xlink:to="loc_us-gaap_InvestmentsFairValueDisclosure_D47247AB295A42CFB70E7D4634796561" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ForeignCurrencyContractAssetFairValueDisclosure" xlink:label="loc_us-gaap_ForeignCurrencyContractAssetFairValueDisclosure_C385C5E7FFD0BA8B436D7D46347AEE0B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_B73284B9B4C48ABA217D7D463478E539" xlink:to="loc_us-gaap_ForeignCurrencyContractAssetFairValueDisclosure_C385C5E7FFD0BA8B436D7D46347AEE0B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PlanAssetsForDeferredCompensationFairValueDisclosure" xlink:label="loc_biib_PlanAssetsForDeferredCompensationFairValueDisclosure_9291B225553A80C3BD3B7D46347A3479" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_B73284B9B4C48ABA217D7D463478E539" xlink:to="loc_biib_PlanAssetsForDeferredCompensationFairValueDisclosure_9291B225553A80C3BD3B7D46347A3479" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsFairValueDisclosure" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_5EC785AAD8D9B61E011C7D46347AE2BC" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_B73284B9B4C48ABA217D7D463478E539" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_5EC785AAD8D9B61E011C7D46347AE2BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_1029B7161240E576B5707D46347A4D0E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisAbstract_FED8DAF3C7A68E56FCD37D4634782106" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_1029B7161240E576B5707D46347A4D0E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure" xlink:label="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_18FEC0E748EF4F31FF1D7D46347ACECB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_1029B7161240E576B5707D46347A4D0E" xlink:to="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_18FEC0E748EF4F31FF1D7D46347ACECB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_D6CBB99546A430B2DA9E7D46347ABAC7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_1029B7161240E576B5707D46347A4D0E" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_D6CBB99546A430B2DA9E7D46347ABAC7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesFairValueDisclosure" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_2ED54D908E3B6AF0024F7D46347BF87D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_1029B7161240E576B5707D46347A4D0E" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_2ED54D908E3B6AF0024F7D46347BF87D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsDetails1" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_506C53F5953D55F4A248C2DB5AD2B2DC" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_FE4CA8E4BE4D5320B2540AFA125DCBEC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_506C53F5953D55F4A248C2DB5AD2B2DC" xlink:to="loc_us-gaap_DebtInstrumentTable_FE4CA8E4BE4D5320B2540AFA125DCBEC" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_8F443E3A532E5775B9562E038CDA1F0F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_FE4CA8E4BE4D5320B2540AFA125DCBEC" xlink:to="loc_us-gaap_DebtInstrumentAxis_8F443E3A532E5775B9562E038CDA1F0F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7DBAC8AC63CB2FA565723C312D147035_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_8F443E3A532E5775B9562E038CDA1F0F" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7DBAC8AC63CB2FA565723C312D147035_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_D9D74108F1CF535C9C0F40051B9C4D09" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_8F443E3A532E5775B9562E038CDA1F0F" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_D9D74108F1CF535C9C0F40051B9C4D09" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_NotesPayableToFumedicaMember" xlink:label="loc_biib_NotesPayableToFumedicaMember_6F08A1D06C035B58B255B82ECEE243B6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_D9D74108F1CF535C9C0F40051B9C4D09" xlink:to="loc_biib_NotesPayableToFumedicaMember_6F08A1D06C035B58B255B82ECEE243B6" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember" xlink:label="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_5B4528E723305B3F8D6372391BF9D77E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_D9D74108F1CF535C9C0F40051B9C4D09" xlink:to="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_5B4528E723305B3F8D6372391BF9D77E" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember" xlink:label="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_7C1703B11BCD5605843B0DD06E69623E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_D9D74108F1CF535C9C0F40051B9C4D09" xlink:to="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_7C1703B11BCD5605843B0DD06E69623E" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" xlink:label="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_AEF3C855C1FD5C6183DA4538E371B217" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_D9D74108F1CF535C9C0F40051B9C4D09" xlink:to="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_AEF3C855C1FD5C6183DA4538E371B217" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" xlink:label="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_E64DE23CA41F5142AC8348534C0D5481" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_D9D74108F1CF535C9C0F40051B9C4D09" xlink:to="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_E64DE23CA41F5142AC8348534C0D5481" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NotesPayableFairValueDisclosure" xlink:label="loc_us-gaap_NotesPayableFairValueDisclosure_27E305DF41A157579B6A1733810894FE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_506C53F5953D55F4A248C2DB5AD2B2DC" xlink:to="loc_us-gaap_NotesPayableFairValueDisclosure_27E305DF41A157579B6A1733810894FE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NotesPayable" xlink:label="loc_us-gaap_NotesPayable_390884966AD65C4C8D5B05CF36F3156A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_506C53F5953D55F4A248C2DB5AD2B2DC" xlink:to="loc_us-gaap_NotesPayable_390884966AD65C4C8D5B05CF36F3156A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentFairValue" xlink:label="loc_us-gaap_DebtInstrumentFairValue_7689E73D288B544A82A519AF798EFA98" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_506C53F5953D55F4A248C2DB5AD2B2DC" xlink:to="loc_us-gaap_DebtInstrumentFairValue_7689E73D288B544A82A519AF798EFA98" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsDetails2" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionContingentConsiderationLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_E08C59E99DABD2CC5173820905344A99" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_6DA3EF66BA49D5778A2482090533DFA8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_E08C59E99DABD2CC5173820905344A99" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_6DA3EF66BA49D5778A2482090533DFA8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_3639A12D5AD2933E9CC182090533D022" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_6DA3EF66BA49D5778A2482090533DFA8" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_3639A12D5AD2933E9CC182090533D022" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_36E1F1F6E111529863C6820905339D10_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_3639A12D5AD2933E9CC182090533D022" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_36E1F1F6E111529863C6820905339D10_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_36E1F1F6E111529863C6820905339D10" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_3639A12D5AD2933E9CC182090533D022" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_36E1F1F6E111529863C6820905339D10" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_E66CF0EB243B88D2E1228209053359AC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_36E1F1F6E111529863C6820905339D10" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_E66CF0EB243B88D2E1228209053359AC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_4965B6BA27A6C45160028209053476D2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_E08C59E99DABD2CC5173820905344A99" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_4965B6BA27A6C45160028209053476D2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_53A38CA67A4B4C5FDDAA820905344DCA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_E08C59E99DABD2CC5173820905344A99" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_53A38CA67A4B4C5FDDAA820905344DCA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_5648D9A14AD990DB05A282090534E482" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_E08C59E99DABD2CC5173820905344A99" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_5648D9A14AD990DB05A282090534E482" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsDetailsTextual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionContingentConsiderationLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_1ACAAFDB9D316D67A8F298F69474F472" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_1ACAAFDB9D316D67A8F298F69474F472" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_AA27181EEDB3CD22821698F69474BCB1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_1ACAAFDB9D316D67A8F298F69474F472" xlink:to="loc_us-gaap_StatementScenarioAxis_AA27181EEDB3CD22821698F69474BCB1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_F1DA0656721B26C5E35F98F69475CEB5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_AA27181EEDB3CD22821698F69474BCB1" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_F1DA0656721B26C5E35F98F69475CEB5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_F1DA0656721B26C5E35F98F69475CEB5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_AA27181EEDB3CD22821698F69474BCB1" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_F1DA0656721B26C5E35F98F69475CEB5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioForecastMember" xlink:label="loc_us-gaap_ScenarioForecastMember_BF3B4169A106210C2D5098F6947585FB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_F1DA0656721B26C5E35F98F69475CEB5" xlink:to="loc_us-gaap_ScenarioForecastMember_BF3B4169A106210C2D5098F6947585FB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_2F84CB1687B8DB3A655798F694753C01" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_1ACAAFDB9D316D67A8F298F69474F472" xlink:to="loc_us-gaap_TypeOfArrangementAxis_2F84CB1687B8DB3A655798F694753C01" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_8B00B190E5643C701EF598F694757833_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_2F84CB1687B8DB3A655798F694753C01" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_8B00B190E5643C701EF598F694757833_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_8B00B190E5643C701EF598F694757833" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_2F84CB1687B8DB3A655798F694753C01" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_8B00B190E5643C701EF598F694757833" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_IonisPharmaceuticalsMember" xlink:label="loc_biib_IonisPharmaceuticalsMember_4E37475C27DB55D78FE798F694762945" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_8B00B190E5643C701EF598F694757833" xlink:to="loc_biib_IonisPharmaceuticalsMember_4E37475C27DB55D78FE798F694762945" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_B9A72A7D2496C0CEE56998F6947682FA" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_1ACAAFDB9D316D67A8F298F69474F472" xlink:to="loc_us-gaap_DebtInstrumentAxis_B9A72A7D2496C0CEE56998F6947682FA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_B9A72A7D2496C0CEE56998F6947682FA" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_B9A72A7D2496C0CEE56998F6947682FA" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_NotesPayableToFumedicaMember" xlink:label="loc_biib_NotesPayableToFumedicaMember_1E32148C40AB665333F198F694773836" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD" xlink:to="loc_biib_NotesPayableToFumedicaMember_1E32148C40AB665333F198F694773836" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember" xlink:label="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_1EDC069C98FBD36E1C7B98F69477E2C4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD" xlink:to="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_1EDC069C98FBD36E1C7B98F69477E2C4" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember" xlink:label="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_8B737D8BA8DA5B0954F698F694776515" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD" xlink:to="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_8B737D8BA8DA5B0954F698F694776515" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" xlink:label="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_668A47D5FBE1B65300DC98F69477AF96" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD" xlink:to="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_668A47D5FBE1B65300DC98F69477AF96" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" xlink:label="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_90D2AA623A8B2DF5B60098F694777DF2" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_CBA2001A77579BE007A398F6947643FD" xlink:to="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_90D2AA623A8B2DF5B60098F694777DF2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetImpairmentCharges" xlink:label="loc_us-gaap_AssetImpairmentCharges_85D6AD2E4E9E740A086F98F69478A456" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:to="loc_us-gaap_AssetImpairmentCharges_85D6AD2E4E9E740A086F98F69478A456" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ServicingAssetAtFairValueDescriptionOfChangesInValuationInputs" xlink:label="loc_us-gaap_ServicingAssetAtFairValueDescriptionOfChangesInValuationInputs_AA36DF84DED17A1734E198F694781C4F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:to="loc_us-gaap_ServicingAssetAtFairValueDescriptionOfChangesInValuationInputs_AA36DF84DED17A1734E198F694781C4F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Termofcollaborationagreement" xlink:label="loc_biib_Termofcollaborationagreement_17598702BF872474A98798F69478E825" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:to="loc_biib_Termofcollaborationagreement_17598702BF872474A98798F69478E825" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_9BF2F1BC7CC52AD67C5298F69478C6FC" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_9BF2F1BC7CC52AD67C5298F69478C6FC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NotesPayable" xlink:label="loc_us-gaap_NotesPayable_59DFA79BB704115F99F998F694797BC8" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:to="loc_us-gaap_NotesPayable_59DFA79BB704115F99F998F694797BC8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_C8BF7DA104E96D39958A98F69479AA82" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_C8BF7DA104E96D39958A98F69479AA82" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_75E61948D12B5566391198F694795128" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_077A0A0E601999FC707F98F69478ED0D" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_75E61948D12B5566391198F694795128" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstruments" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsLineItems" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_F9FF47B0DFDC5788AC14991BD9CBF067" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_1241104C56B751488FED5AC8A6F27E63" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_F9FF47B0DFDC5788AC14991BD9CBF067" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_1241104C56B751488FED5AC8A6F27E63" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentTypeAxis" xlink:label="loc_us-gaap_InvestmentTypeAxis_97AFC8DD31235B0A9B05F013F6F0B0C0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_1241104C56B751488FED5AC8A6F27E63" xlink:to="loc_us-gaap_InvestmentTypeAxis_97AFC8DD31235B0A9B05F013F6F0B0C0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_A6B969E6CDEA712886F73C312B6A2E5F_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_97AFC8DD31235B0A9B05F013F6F0B0C0" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_A6B969E6CDEA712886F73C312B6A2E5F_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_49D6C2F2044457D49E685EE2B88063DC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_97AFC8DD31235B0A9B05F013F6F0B0C0" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_49D6C2F2044457D49E685EE2B88063DC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommercialPaperMember" xlink:label="loc_us-gaap_CommercialPaperMember_BA5C654D30BA5A45877676DFC52BCDB6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_49D6C2F2044457D49E685EE2B88063DC" xlink:to="loc_us-gaap_CommercialPaperMember_BA5C654D30BA5A45877676DFC52BCDB6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RepurchaseAgreementsMember" xlink:label="loc_us-gaap_RepurchaseAgreementsMember_B721325493C05C66B2CD6FC511176F09" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_49D6C2F2044457D49E685EE2B88063DC" xlink:to="loc_us-gaap_RepurchaseAgreementsMember_B721325493C05C66B2CD6FC511176F09" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="loc_us-gaap_MoneyMarketFundsMember_1A1BA8B4138C57BD84047A2AAD9AEB69" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_49D6C2F2044457D49E685EE2B88063DC" xlink:to="loc_us-gaap_MoneyMarketFundsMember_1A1BA8B4138C57BD84047A2AAD9AEB69" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtSecuritiesMember" xlink:label="loc_us-gaap_DebtSecuritiesMember_00D14DB8F848541E8CF1837844D1E0B4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_49D6C2F2044457D49E685EE2B88063DC" xlink:to="loc_us-gaap_DebtSecuritiesMember_00D14DB8F848541E8CF1837844D1E0B4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashEquivalentsAtCarryingValueAbstract" xlink:label="loc_us-gaap_CashEquivalentsAtCarryingValueAbstract_DFE9F4884D885C8E81794EF16D34840B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_F9FF47B0DFDC5788AC14991BD9CBF067" xlink:to="loc_us-gaap_CashEquivalentsAtCarryingValueAbstract_DFE9F4884D885C8E81794EF16D34840B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashEquivalentsAtCarryingValue_BD4048014F445E9DB89DDF254EC7F40B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashEquivalentsAtCarryingValueAbstract_DFE9F4884D885C8E81794EF16D34840B" xlink:to="loc_us-gaap_CashEquivalentsAtCarryingValue_BD4048014F445E9DB89DDF254EC7F40B" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetails1" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_68DA4ED70F805BE6AEA323F09EA79B6D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_68DA4ED70F805BE6AEA323F09EA79B6D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="loc_us-gaap_FinancialInstrumentAxis_EADCB5FAEAD95D75BBD0B0CB5FE5B62C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_68DA4ED70F805BE6AEA323F09EA79B6D" xlink:to="loc_us-gaap_FinancialInstrumentAxis_EADCB5FAEAD95D75BBD0B0CB5FE5B62C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_083A1E0C7C17F420BAA3B821C66AE90F_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_EADCB5FAEAD95D75BBD0B0CB5FE5B62C" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_083A1E0C7C17F420BAA3B821C66AE90F_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_EADCB5FAEAD95D75BBD0B0CB5FE5B62C" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_CorporateDebtSecuritiesCurrentMember" xlink:label="loc_biib_CorporateDebtSecuritiesCurrentMember_CB7D1A14B301539E8F626B3B02FD6E45" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:to="loc_biib_CorporateDebtSecuritiesCurrentMember_CB7D1A14B301539E8F626B3B02FD6E45" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_CorporateDebtSecuritiesNonCurrentMember" xlink:label="loc_biib_CorporateDebtSecuritiesNonCurrentMember_D6B01F09F3505C5399325DFB0A5B54AD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:to="loc_biib_CorporateDebtSecuritiesNonCurrentMember_D6B01F09F3505C5399325DFB0A5B54AD" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_GovernmentSecuritiesCurrentMember" xlink:label="loc_biib_GovernmentSecuritiesCurrentMember_35F26DFBAB3E5D058181258FB3853650" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:to="loc_biib_GovernmentSecuritiesCurrentMember_35F26DFBAB3E5D058181258FB3853650" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_GovernmentSecuritiesNonCurrentMember" xlink:label="loc_biib_GovernmentSecuritiesNonCurrentMember_F7E9CAC6186C5E5E85FB8EAD88C92449" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:to="loc_biib_GovernmentSecuritiesNonCurrentMember_F7E9CAC6186C5E5E85FB8EAD88C92449" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember" xlink:label="loc_biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember_A36577C43BCF528A857CCEEAAD711B3D" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:to="loc_biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember_A36577C43BCF528A857CCEEAAD711B3D" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember" xlink:label="loc_biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember_46CF007E30F95CB088E6571DF7414A72" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:to="loc_biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember_46CF007E30F95CB088E6571DF7414A72" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquitySecuritiesMember" xlink:label="loc_us-gaap_EquitySecuritiesMember_9CD38DFBF3765C8F83C022C8FE90944A" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4F98495F60F95F8FACE2935942AD3AC5" xlink:to="loc_us-gaap_EquitySecuritiesMember_9CD38DFBF3765C8F83C022C8FE90944A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_9AFDC4DC4AFA57879E750871CD014470" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_9AFDC4DC4AFA57879E750871CD014470" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_05A7B3ECF5D05540B687264FD2F08B7E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_05A7B3ECF5D05540B687264FD2F08B7E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_75AD611089955CD9807610F0F5587016" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_75AD611089955CD9807610F0F5587016" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_DAE8CE0EEF6A5CB396D61E7BE135122B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_DAE8CE0EEF6A5CB396D61E7BE135122B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecurities_CCA6EDEB3339500782A6838C8467D45A" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_AvailableForSaleSecurities_CCA6EDEB3339500782A6838C8467D45A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_E37C2FABB1DB5EB1BF27D0EDFFA6E2A6" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_E37C2FABB1DB5EB1BF27D0EDFFA6E2A6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_CBBE45561D105A6794482743B016BCC7" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_CBBE45561D105A6794482743B016BCC7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAmortizedCost" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAmortizedCost_48F25035F9BF500F99404D2AA05A94BC" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_488E6725A0C5571C9BB298A97CA33745" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAmortizedCost_48F25035F9BF500F99404D2AA05A94BC" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetails2" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetails3" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetailsTextual" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsFinancialInstrumentsDetailsTextual2" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_52256790C3B9504F9638FBECCA0AC95B" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C90D560C4D3A59169888694C746B2622" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_52256790C3B9504F9638FBECCA0AC95B" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C90D560C4D3A59169888694C746B2622" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_C6CD941A0FD25C0CBF52939F231A3E9C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C90D560C4D3A59169888694C746B2622" xlink:to="loc_us-gaap_TypeOfArrangementAxis_C6CD941A0FD25C0CBF52939F231A3E9C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_9770AA6F34A42D698C07AD5D1BF76D48_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_C6CD941A0FD25C0CBF52939F231A3E9C" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_9770AA6F34A42D698C07AD5D1BF76D48_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_888DF3A333175CB095EDE6B8FB21DD7B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_C6CD941A0FD25C0CBF52939F231A3E9C" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_888DF3A333175CB095EDE6B8FB21DD7B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_IonisPharmaceuticalsMember" xlink:label="loc_biib_IonisPharmaceuticalsMember_B6AE7C953FD15FBA958FB4F127DF2ECA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_888DF3A333175CB095EDE6B8FB21DD7B" xlink:to="loc_biib_IonisPharmaceuticalsMember_B6AE7C953FD15FBA958FB4F127DF2ECA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="loc_us-gaap_FinancialInstrumentAxis_CCC1FE8966135A9582C5A5D452930713" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C90D560C4D3A59169888694C746B2622" xlink:to="loc_us-gaap_FinancialInstrumentAxis_CCC1FE8966135A9582C5A5D452930713" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_FFA3F80D53E33DAEE308AD4DBDA56B1A_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_CCC1FE8966135A9582C5A5D452930713" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_FFA3F80D53E33DAEE308AD4DBDA56B1A_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_E62178FF105A5714B115E90C146E4A6E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_CCC1FE8966135A9582C5A5D452930713" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_E62178FF105A5714B115E90C146E4A6E" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_StrategicInvestmentsMember" xlink:label="loc_biib_StrategicInvestmentsMember_58B3FFD426345543800DEC7FABD740ED" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_E62178FF105A5714B115E90C146E4A6E" xlink:to="loc_biib_StrategicInvestmentsMember_58B3FFD426345543800DEC7FABD740ED" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_StrategicInvestmentPortfolio" xlink:label="loc_biib_StrategicInvestmentPortfolio_18B4487A5AD05592909D7A90B36F0B40" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_52256790C3B9504F9638FBECCA0AC95B" xlink:to="loc_biib_StrategicInvestmentPortfolio_18B4487A5AD05592909D7A90B36F0B40" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Termofcollaborationagreement" xlink:label="loc_biib_Termofcollaborationagreement_E699B9FAC2795BF898AC0133ADDB381D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_52256790C3B9504F9638FBECCA0AC95B" xlink:to="loc_biib_Termofcollaborationagreement_E699B9FAC2795BF898AC0133ADDB381D" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Totalpaymenttoentercollaborationagreement" xlink:label="loc_biib_Totalpaymenttoentercollaborationagreement_D74CF6EC1D6C5844AB0B6B197599BE85" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_52256790C3B9504F9638FBECCA0AC95B" xlink:to="loc_biib_Totalpaymenttoentercollaborationagreement_D74CF6EC1D6C5844AB0B6B197599BE85" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Upfrontpaymentforcollaborationagreement" xlink:label="loc_biib_Upfrontpaymentforcollaborationagreement_0B78D49A3D21535EAF9081EB9F55B9CD" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_52256790C3B9504F9638FBECCA0AC95B" xlink:to="loc_biib_Upfrontpaymentforcollaborationagreement_0B78D49A3D21535EAF9081EB9F55B9CD" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Investmentincommonstocksharespurchased" xlink:label="loc_biib_Investmentincommonstocksharespurchased_BEEE074C6F055D1C8952AAFC6A73F0AA" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_52256790C3B9504F9638FBECCA0AC95B" xlink:to="loc_biib_Investmentincommonstocksharespurchased_BEEE074C6F055D1C8952AAFC6A73F0AA" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Purchaseofcommonstock" xlink:label="loc_biib_Purchaseofcommonstock_C51D03CC9886551DA974233EE5605644" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_52256790C3B9504F9638FBECCA0AC95B" xlink:to="loc_biib_Purchaseofcommonstock_C51D03CC9886551DA974233EE5605644" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxes" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesDetailsTextual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxContingencyLineItems" xlink:label="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxContingencyTable" xlink:label="loc_us-gaap_IncomeTaxContingencyTable_4D9B4A723E574D2DCAD68DC0E43AAE27" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_IncomeTaxContingencyTable_4D9B4A723E574D2DCAD68DC0E43AAE27" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_9FA4FFFF3DFB1989CC078DC0E43B2BB9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_4D9B4A723E574D2DCAD68DC0E43AAE27" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_9FA4FFFF3DFB1989CC078DC0E43B2BB9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_5A7A0620E2AED00B12548DC0E43C1F01_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_9FA4FFFF3DFB1989CC078DC0E43B2BB9" xlink:to="loc_us-gaap_TypeOfAdoptionMember_5A7A0620E2AED00B12548DC0E43C1F01_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_5A7A0620E2AED00B12548DC0E43C1F01" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_9FA4FFFF3DFB1989CC078DC0E43B2BB9" xlink:to="loc_us-gaap_TypeOfAdoptionMember_5A7A0620E2AED00B12548DC0E43C1F01" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountingStandardsUpdate201616Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201616Member_4DE5E71AC3E811D3DE7E8DC0E43C018F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_5A7A0620E2AED00B12548DC0E43C1F01" xlink:to="loc_us-gaap_AccountingStandardsUpdate201616Member_4DE5E71AC3E811D3DE7E8DC0E43C018F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_AB7B37B9442023A05B3B8DC0E43D4905" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_4D9B4A723E574D2DCAD68DC0E43AAE27" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_AB7B37B9442023A05B3B8DC0E43D4905" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_54452BDAF85CC90BB9808DC0E43D6051_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_AB7B37B9442023A05B3B8DC0E43D4905" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_54452BDAF85CC90BB9808DC0E43D6051_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_54452BDAF85CC90BB9808DC0E43D6051" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_AB7B37B9442023A05B3B8DC0E43D4905" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_54452BDAF85CC90BB9808DC0E43D6051" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxAssetDomain" xlink:label="loc_us-gaap_DeferredTaxAssetDomain_3FBBDBDA5BBAAF392D2E8DC0E43DA908" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_54452BDAF85CC90BB9808DC0E43D6051" xlink:to="loc_us-gaap_DeferredTaxAssetDomain_3FBBDBDA5BBAAF392D2E8DC0E43DA908" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DeferredtaxliabilityMember" xlink:label="loc_biib_DeferredtaxliabilityMember_376FC843F40E38D71CE98DC0E43EC64F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_54452BDAF85CC90BB9808DC0E43D6051" xlink:to="loc_biib_DeferredtaxliabilityMember_376FC843F40E38D71CE98DC0E43EC64F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_846CD57D1F02B8BF7A818DC0E43EE1FD" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_54452BDAF85CC90BB9808DC0E43D6051" xlink:to="loc_us-gaap_RetainedEarningsMember_846CD57D1F02B8BF7A818DC0E43EE1FD" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ForeignearningstypeAxis" xlink:label="loc_biib_ForeignearningstypeAxis_E0B28FC383A0CCA6181F8DC0E43E68BC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_4D9B4A723E574D2DCAD68DC0E43AAE27" xlink:to="loc_biib_ForeignearningstypeAxis_E0B28FC383A0CCA6181F8DC0E43E68BC" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ForeignearningstypeDomain" xlink:label="loc_biib_ForeignearningstypeDomain_7B0FD818B10B61B678618DC0E43E31F4_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_ForeignearningstypeAxis_E0B28FC383A0CCA6181F8DC0E43E68BC" xlink:to="loc_biib_ForeignearningstypeDomain_7B0FD818B10B61B678618DC0E43E31F4_default" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ForeignearningstypeDomain" xlink:label="loc_biib_ForeignearningstypeDomain_7B0FD818B10B61B678618DC0E43E31F4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_ForeignearningstypeAxis_E0B28FC383A0CCA6181F8DC0E43E68BC" xlink:to="loc_biib_ForeignearningstypeDomain_7B0FD818B10B61B678618DC0E43E31F4" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ForeignearningsintheformofcashandcashequivalentsMember" xlink:label="loc_biib_ForeignearningsintheformofcashandcashequivalentsMember_E4B90702AC41D58D65F88DC0E43E462C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ForeignearningstypeDomain_7B0FD818B10B61B678618DC0E43E31F4" xlink:to="loc_biib_ForeignearningsintheformofcashandcashequivalentsMember_E4B90702AC41D58D65F88DC0E43E462C" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_OtherForeignEarningsMember" xlink:label="loc_biib_OtherForeignEarningsMember_4FE1CE09DF8900B353C28DC0E43F261E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ForeignearningstypeDomain_7B0FD818B10B61B678618DC0E43E31F4" xlink:to="loc_biib_OtherForeignEarningsMember_4FE1CE09DF8900B353C28DC0E43F261E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_DD53C145118EC460914E8DC0E43F81A9" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_4D9B4A723E574D2DCAD68DC0E43AAE27" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_DD53C145118EC460914E8DC0E43F81A9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_677052567F51A81138AB8DC0E43FBC39_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_DD53C145118EC460914E8DC0E43F81A9" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_677052567F51A81138AB8DC0E43FBC39_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_677052567F51A81138AB8DC0E43FBC39" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_DD53C145118EC460914E8DC0E43F81A9" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_677052567F51A81138AB8DC0E43FBC39" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_U.S.2017TaxActMember" xlink:label="loc_biib_U.S.2017TaxActMember_EE6D16D082BBC3955C928DC0E43F49DC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_677052567F51A81138AB8DC0E43FBC39" xlink:to="loc_biib_U.S.2017TaxActMember_EE6D16D082BBC3955C928DC0E43F49DC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ForeignCountryMember" xlink:label="loc_us-gaap_ForeignCountryMember_ADA066D760169A0C0D748DC0E43F636A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_677052567F51A81138AB8DC0E43FBC39" xlink:to="loc_us-gaap_ForeignCountryMember_ADA066D760169A0C0D748DC0E43F636A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_9463FB40373B549525E68DC0E440B4AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_9463FB40373B549525E68DC0E440B4AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_A0E7B70BBC267B74D06F8DC0E440DE5A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_A0E7B70BBC267B74D06F8DC0E440DE5A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings_3EFCE8567D58B10A073B8DC0E440D266" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings_3EFCE8567D58B10A073B8DC0E440D266" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentOfDeferredTaxAssetLiability" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentOfDeferredTaxAssetLiability_200198167A6B16DB8F1A8DC0E4419E17" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentOfDeferredTaxAssetLiability_200198167A6B16DB8F1A8DC0E4419E17" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherTaxExpenseBenefit" xlink:label="loc_us-gaap_OtherTaxExpenseBenefit_D44196E20D75F038B62D8DC33174DF11" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_OtherTaxExpenseBenefit_D44196E20D75F038B62D8DC33174DF11" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TransitionTollTaxRepatriationTaxRate" xlink:label="loc_biib_TransitionTollTaxRepatriationTaxRate_07E0C4F59AD7D9C2D5838DC0E441D37A" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_biib_TransitionTollTaxRepatriationTaxRate_07E0C4F59AD7D9C2D5838DC0E441D37A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxLiabilitiesGrossNoncurrent" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesGrossNoncurrent_2886160702099A6B0B8A8DC0E441250D" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesGrossNoncurrent_2886160702099A6B0B8A8DC0E441250D" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PaymentforTransitionTollTax" xlink:label="loc_biib_PaymentforTransitionTollTax_4D289CCC6AA878D3512A8DC6E19FA9FF" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_biib_PaymentforTransitionTollTax_4D289CCC6AA878D3512A8DC6E19FA9FF" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DeferredTaxLiabilitiesDecreaseDuetoOverpayment" xlink:label="loc_biib_DeferredTaxLiabilitiesDecreaseDuetoOverpayment_66D5ED2FEBD5E08DEF848DCD20663F02" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_biib_DeferredTaxLiabilitiesDecreaseDuetoOverpayment_66D5ED2FEBD5E08DEF848DCD20663F02" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_F07A10222DD47788720D8DC0E4413E39" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_F07A10222DD47788720D8DC0E4413E39" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued" xlink:label="loc_us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued_FADE9076316CABBD811D8DC0E441F67E" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_33283B98D68E709882C78DC0E4407B24" xlink:to="loc_us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued_FADE9076316CABBD811D8DC0E441F67E" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwill" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_B399CE344E609484688698EB58DA8B4D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_197BF4CC7DE3D31237C898EB58D4A2F9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_B399CE344E609484688698EB58DA8B4D" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_197BF4CC7DE3D31237C898EB58D4A2F9" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ResearchanddevelopmentassetAxis" xlink:label="loc_biib_ResearchanddevelopmentassetAxis_E4A728E74F1D218CBA1F98EB58D433DE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_197BF4CC7DE3D31237C898EB58D4A2F9" xlink:to="loc_biib_ResearchanddevelopmentassetAxis_E4A728E74F1D218CBA1F98EB58D433DE" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ResearchanddevelopmentassetDomain" xlink:label="loc_biib_ResearchanddevelopmentassetDomain_04D97F96EDD49700C4CB98EB58D4667A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_ResearchanddevelopmentassetAxis_E4A728E74F1D218CBA1F98EB58D433DE" xlink:to="loc_biib_ResearchanddevelopmentassetDomain_04D97F96EDD49700C4CB98EB58D4667A_default" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ResearchanddevelopmentassetDomain" xlink:label="loc_biib_ResearchanddevelopmentassetDomain_04D97F96EDD49700C4CB98EB58D4667A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_ResearchanddevelopmentassetAxis_E4A728E74F1D218CBA1F98EB58D433DE" xlink:to="loc_biib_ResearchanddevelopmentassetDomain_04D97F96EDD49700C4CB98EB58D4667A" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PLSRMember" xlink:label="loc_biib_PLSRMember_198F3D3ECA8213DBC60298EB58D4FC70" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ResearchanddevelopmentassetDomain_04D97F96EDD49700C4CB98EB58D4667A" xlink:to="loc_biib_PLSRMember_198F3D3ECA8213DBC60298EB58D4FC70" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TGNMember" xlink:label="loc_biib_TGNMember_D323692687EB29BD169598EB58D5D225" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ResearchanddevelopmentassetDomain_04D97F96EDD49700C4CB98EB58D4667A" xlink:to="loc_biib_TGNMember_D323692687EB29BD169598EB58D5D225" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_4C6CEB8C444C23D50E3A98EB58D51A2A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_197BF4CC7DE3D31237C898EB58D4A2F9" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_4C6CEB8C444C23D50E3A98EB58D51A2A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_CA3B751F71F78A18000F98EB58D528EE_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_4C6CEB8C444C23D50E3A98EB58D51A2A" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_CA3B751F71F78A18000F98EB58D528EE_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_CA3B751F71F78A18000F98EB58D528EE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_4C6CEB8C444C23D50E3A98EB58D51A2A" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_CA3B751F71F78A18000F98EB58D528EE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InProcessResearchAndDevelopmentMember" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_A9F0EED7317BE94F9F2C98EB58D5D412" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_CA3B751F71F78A18000F98EB58D528EE" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_A9F0EED7317BE94F9F2C98EB58D5D412" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TrademarksAndTradeNamesMember" xlink:label="loc_us-gaap_TrademarksAndTradeNamesMember_5C35239C3E9D97093D7E98EB58D61781" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_CA3B751F71F78A18000F98EB58D528EE" xlink:to="loc_us-gaap_TrademarksAndTradeNamesMember_5C35239C3E9D97093D7E98EB58D61781" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_108D14B371095152023998EB58D63795" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_197BF4CC7DE3D31237C898EB58D4A2F9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_108D14B371095152023998EB58D63795" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_342F189B5E77198ED60E98EB58D69CB6_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_108D14B371095152023998EB58D63795" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_342F189B5E77198ED60E98EB58D69CB6_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_342F189B5E77198ED60E98EB58D69CB6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_108D14B371095152023998EB58D63795" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_342F189B5E77198ED60E98EB58D69CB6" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_OutLicensedPatentsMember" xlink:label="loc_biib_OutLicensedPatentsMember_EC230354593BA0A378DF98EB58D6C78B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_342F189B5E77198ED60E98EB58D69CB6" xlink:to="loc_biib_OutLicensedPatentsMember_EC230354593BA0A378DF98EB58D6C78B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_886407F52941ED630DAA98EB58D7B183" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_342F189B5E77198ED60E98EB58D69CB6" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_886407F52941ED630DAA98EB58D7B183" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_InLicensedPatentsMember" xlink:label="loc_biib_InLicensedPatentsMember_FFCEA7498C874C57764098EB58D7B643" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_342F189B5E77198ED60E98EB58D69CB6" xlink:to="loc_biib_InLicensedPatentsMember_FFCEA7498C874C57764098EB58D7B643" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_2A23F62D3DA6183BDD9A98EB58D77C9E" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_197BF4CC7DE3D31237C898EB58D4A2F9" xlink:to="loc_srt_ProductOrServiceAxis_2A23F62D3DA6183BDD9A98EB58D77C9E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_9DD52A77CC8780086BE798EB58D7308B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_2A23F62D3DA6183BDD9A98EB58D77C9E" xlink:to="loc_srt_ProductsAndServicesDomain_9DD52A77CC8780086BE798EB58D7308B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_9DD52A77CC8780086BE798EB58D7308B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_2A23F62D3DA6183BDD9A98EB58D77C9E" xlink:to="loc_srt_ProductsAndServicesDomain_9DD52A77CC8780086BE798EB58D7308B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_AVONEXMember" xlink:label="loc_biib_AVONEXMember_79C358945818C891E1C398EB58D7A13C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_9DD52A77CC8780086BE798EB58D7308B" xlink:to="loc_biib_AVONEXMember_79C358945818C891E1C398EB58D7A13C" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TysabriProductMember" xlink:label="loc_biib_TysabriProductMember_28F034C7672966EF029C98EB58D84C8A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_9DD52A77CC8780086BE798EB58D7308B" xlink:to="loc_biib_TysabriProductMember_28F034C7672966EF029C98EB58D84C8A" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TecfideraMember" xlink:label="loc_biib_TecfideraMember_4B3AC6CE416EC5FC3F1D98EB58D89611" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_9DD52A77CC8780086BE798EB58D7308B" xlink:to="loc_biib_TecfideraMember_4B3AC6CE416EC5FC3F1D98EB58D89611" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_FDE2AEE8BFAD02E97F4B98EB58D8D760" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_197BF4CC7DE3D31237C898EB58D4A2F9" xlink:to="loc_srt_RangeAxis_FDE2AEE8BFAD02E97F4B98EB58D8D760" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_56345908FD3AE631058C98EB58D8C5E3_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_FDE2AEE8BFAD02E97F4B98EB58D8D760" xlink:to="loc_srt_RangeMember_56345908FD3AE631058C98EB58D8C5E3_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_56345908FD3AE631058C98EB58D8C5E3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_FDE2AEE8BFAD02E97F4B98EB58D8D760" xlink:to="loc_srt_RangeMember_56345908FD3AE631058C98EB58D8C5E3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_A7E8AEB0AA693D8278EA98EB58DA0847" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_56345908FD3AE631058C98EB58D8C5E3" xlink:to="loc_srt_MinimumMember_A7E8AEB0AA693D8278EA98EB58DA0847" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_F3A6C9FC11254819151298EB58DAE816" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_56345908FD3AE631058C98EB58D8C5E3" xlink:to="loc_srt_MaximumMember_F3A6C9FC11254819151298EB58DAE816" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_B399CE344E609484688698EB58DA8B4D" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_336AB69D87E7858CE4F498EB58DAACFB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_336AB69D87E7858CE4F498EB58DAACFB" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_IndefiniteLivedIntangibleAssetsUsefulLife" xlink:label="loc_biib_IndefiniteLivedIntangibleAssetsUsefulLife_D72B01C7B2314F51832298EB58DB58AF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:to="loc_biib_IndefiniteLivedIntangibleAssetsUsefulLife_D72B01C7B2314F51832298EB58DB58AF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_42AF46D116136323200398EB58DBB9F5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_42AF46D116136323200398EB58DBB9F5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_18B0EB4710499A19819798EB58DB8BF4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_18B0EB4710499A19819798EB58DB8BF4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_DF145A522EE089EE753998EB58DC070C" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_DF145A522EE089EE753998EB58DC070C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_3B4DEA1643DE3099A51A98EB58DC3626" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_3B4DEA1643DE3099A51A98EB58DC3626" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_25E88468907CF8A516D398EB58DCBC01" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_25E88468907CF8A516D398EB58DCBC01" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_95C1588A43567167155E98EB58DC3F22" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_622DE623CAEB6F694C5298EB58DA1F48" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_95C1588A43567167155E98EB58DC3F22" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_E0E0A9E410E9D72711F998F18EC9D9E1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_B399CE344E609484688698EB58DA8B4D" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_E0E0A9E410E9D72711F998F18EC9D9E1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_C970D59D424500AA426C98EB58DC5AB5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_B399CE344E609484688698EB58DA8B4D" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_C970D59D424500AA426C98EB58DC5AB5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LitigationSettlementAmountAwardedToOtherParty" xlink:label="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_B5C22868FDE126A62AB798EB58DCD997" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_B399CE344E609484688698EB58DA8B4D" xlink:to="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_B5C22868FDE126A62AB798EB58DCD997" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails1" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="loc_us-gaap_GoodwillLineItems_6E5D4D8A1AB8EA76B7CB820905851338" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfGoodwillTable" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_E772B71435FD38FA6CB9820905830BFF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_6E5D4D8A1AB8EA76B7CB820905851338" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_E772B71435FD38FA6CB9820905830BFF" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_57BFF1084E163BD1BA7B82090584E863" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_E772B71435FD38FA6CB9820905830BFF" xlink:to="loc_srt_ProductOrServiceAxis_57BFF1084E163BD1BA7B82090584E863" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_CE83F36C6D23F889586182090584050A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_57BFF1084E163BD1BA7B82090584E863" xlink:to="loc_srt_ProductsAndServicesDomain_CE83F36C6D23F889586182090584050A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_CE83F36C6D23F889586182090584050A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_57BFF1084E163BD1BA7B82090584E863" xlink:to="loc_srt_ProductsAndServicesDomain_CE83F36C6D23F889586182090584050A" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TecfideraMember" xlink:label="loc_biib_TecfideraMember_0C6FC5F2E7B733D7589D82090584C931" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_CE83F36C6D23F889586182090584050A" xlink:to="loc_biib_TecfideraMember_0C6FC5F2E7B733D7589D82090584C931" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_C50AD516872C9EB32C6C820905842B06" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_E772B71435FD38FA6CB9820905830BFF" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_C50AD516872C9EB32C6C820905842B06" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_15E7A47F930AC4CE748382090585F8ED_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_C50AD516872C9EB32C6C820905842B06" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_15E7A47F930AC4CE748382090585F8ED_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_15E7A47F930AC4CE748382090585F8ED" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_C50AD516872C9EB32C6C820905842B06" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_15E7A47F930AC4CE748382090585F8ED" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FumapharmAgMember" xlink:label="loc_biib_FumapharmAgMember_C1FE8A6CF56250F7FA568209058507AA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_15E7A47F930AC4CE748382090585F8ED" xlink:to="loc_biib_FumapharmAgMember_C1FE8A6CF56250F7FA568209058507AA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_129151D6B26E01A3D271820905852EE1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_6E5D4D8A1AB8EA76B7CB820905851338" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_129151D6B26E01A3D271820905852EE1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_A7E6C0BD1D0E465B955F820905866FF7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_6E5D4D8A1AB8EA76B7CB820905851338" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_A7E6C0BD1D0E465B955F820905866FF7" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/Inventory" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/InventoryDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/InventoryTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/InvestmentsInVariableInterestEntities" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/InvestmentsInVariableInterestEntitiesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_VariableInterestEntityLineItems" xlink:label="loc_us-gaap_VariableInterestEntityLineItems_B4146CDF79995B5CB369C1F76784133E" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable" xlink:label="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_9E45ED2B7A46526A9AE0AC3AC69B2807" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_B4146CDF79995B5CB369C1F76784133E" xlink:to="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_9E45ED2B7A46526A9AE0AC3AC69B2807" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis" xlink:label="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_F2914C24276E503A85F1407A7FF9FC04" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_9E45ED2B7A46526A9AE0AC3AC69B2807" xlink:to="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_F2914C24276E503A85F1407A7FF9FC04" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassificationOfVariableInterestEntityDomain" xlink:label="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_8F35623253CFB67413C4C1F00D002250_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_F2914C24276E503A85F1407A7FF9FC04" xlink:to="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_8F35623253CFB67413C4C1F00D002250_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassificationOfVariableInterestEntityDomain" xlink:label="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_736A48101DD05E218FA7341A64003BE2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_F2914C24276E503A85F1407A7FF9FC04" xlink:to="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_736A48101DD05E218FA7341A64003BE2" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_NeurimmuneMember" xlink:label="loc_biib_NeurimmuneMember_1F54F4E657165CE8996E01CCC6F9BF5B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_736A48101DD05E218FA7341A64003BE2" xlink:to="loc_biib_NeurimmuneMember_1F54F4E657165CE8996E01CCC6F9BF5B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaymentsToMinorityShareholders" xlink:label="loc_us-gaap_PaymentsToMinorityShareholders_E4A904676C455FE29E56644EFF3C5382" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_B4146CDF79995B5CB369C1F76784133E" xlink:to="loc_us-gaap_PaymentsToMinorityShareholders_E4A904676C455FE29E56644EFF3C5382" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Reductioninroyaltyratepayableoncommercialsales" xlink:label="loc_biib_Reductioninroyaltyratepayableoncommercialsales_5F68ADDEF1C35FB49D66431360A172FE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_B4146CDF79995B5CB369C1F76784133E" xlink:to="loc_biib_Reductioninroyaltyratepayableoncommercialsales_5F68ADDEF1C35FB49D66431360A172FE" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Additionalreductioninroyaltyratepayableoncommercialsales" xlink:label="loc_biib_Additionalreductioninroyaltyratepayableoncommercialsales_FE3B70A2D4DF5614B7D993A64A22113F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_B4146CDF79995B5CB369C1F76784133E" xlink:to="loc_biib_Additionalreductioninroyaltyratepayableoncommercialsales_FE3B70A2D4DF5614B7D993A64A22113F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_InvestmentInVariableInterestEntitiesTextualAbstract" xlink:label="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_ECADB44A7C6A57E7B07C377C7D28BFC0" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_B4146CDF79995B5CB369C1F76784133E" xlink:to="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_ECADB44A7C6A57E7B07C377C7D28BFC0" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities" xlink:label="loc_biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities_86CBAB1957EB516AB3607BB30C769C8C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_ECADB44A7C6A57E7B07C377C7D28BFC0" xlink:to="loc_biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities_86CBAB1957EB516AB3607BB30C769C8C" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/Litigation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/LitigationLitigationDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxContingencyLineItems" xlink:label="loc_us-gaap_IncomeTaxContingencyLineItems_C5998159651E55E8B8DFF1B92CD8B2BF" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxContingencyTable" xlink:label="loc_us-gaap_IncomeTaxContingencyTable_E6D67F1949E75553B79EC288D22BA345" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_C5998159651E55E8B8DFF1B92CD8B2BF" xlink:to="loc_us-gaap_IncomeTaxContingencyTable_E6D67F1949E75553B79EC288D22BA345" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_FA07562EE95E5EEE97DF3F2B1894EA04" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_E6D67F1949E75553B79EC288D22BA345" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_FA07562EE95E5EEE97DF3F2B1894EA04" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_577A86CF865121A58D593C312BF61A03_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_FA07562EE95E5EEE97DF3F2B1894EA04" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_577A86CF865121A58D593C312BF61A03_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_2BBEA2D28B4C5D42995845EBE8C5144B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_FA07562EE95E5EEE97DF3F2B1894EA04" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_2BBEA2D28B4C5D42995845EBE8C5144B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_BR" xlink:label="loc_country_BR_842A9F6B8215574494B5EB412DF36094" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_2BBEA2D28B4C5D42995845EBE8C5144B" xlink:to="loc_country_BR_842A9F6B8215574494B5EB412DF36094" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingencyDamagesSought" xlink:label="loc_us-gaap_LossContingencyDamagesSought_E608ACFF276B596594B3A1F0A3B5ECBA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_C5998159651E55E8B8DFF1B92CD8B2BF" xlink:to="loc_us-gaap_LossContingencyDamagesSought_E608ACFF276B596594B3A1F0A3B5ECBA" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_530888FA94CD1FDE43C37E4586F9BE02" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_1891EF7538BFCC5BA8FB7E4586F9545B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_530888FA94CD1FDE43C37E4586F9BE02" xlink:to="loc_us-gaap_StatementTable_1891EF7538BFCC5BA8FB7E4586F9545B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_3C5352C640113ADF6CB67E4587184D61" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_1891EF7538BFCC5BA8FB7E4586F9545B" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_3C5352C640113ADF6CB67E4587184D61" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_60FB5DE9ABB06F9003F67E458718987F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_3C5352C640113ADF6CB67E4587184D61" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_60FB5DE9ABB06F9003F67E458718987F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_60FB5DE9ABB06F9003F67E458718987F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_3C5352C640113ADF6CB67E4587184D61" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_60FB5DE9ABB06F9003F67E458718987F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCurrentLiabilitiesMember" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_91773D1A6763059A507A7E4587470ECA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_60FB5DE9ABB06F9003F67E458718987F" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_91773D1A6763059A507A7E4587470ECA" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailOtherConsolidatedFinancialStatementDetailsTextual" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipment" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipmentDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_ECCC302C81AF5278B02F9614EEA1E3ED" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_71F831D85D085315B370520802859BEC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_ECCC302C81AF5278B02F9614EEA1E3ED" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_71F831D85D085315B370520802859BEC" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FacilityLocationAxis" xlink:label="loc_biib_FacilityLocationAxis_7D1941C08E125BEF8DE54CE17E8F0F80" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_71F831D85D085315B370520802859BEC" xlink:to="loc_biib_FacilityLocationAxis_7D1941C08E125BEF8DE54CE17E8F0F80" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FacilityLocationDomain" xlink:label="loc_biib_FacilityLocationDomain_AB18FBBB5465C8B58B313C312D52CC0D_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_FacilityLocationAxis_7D1941C08E125BEF8DE54CE17E8F0F80" xlink:to="loc_biib_FacilityLocationDomain_AB18FBBB5465C8B58B313C312D52CC0D_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FacilityLocationDomain" xlink:label="loc_biib_FacilityLocationDomain_2E855BF3CCCC5CAFA6E0E2ACD573E5A3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_FacilityLocationAxis_7D1941C08E125BEF8DE54CE17E8F0F80" xlink:to="loc_biib_FacilityLocationDomain_2E855BF3CCCC5CAFA6E0E2ACD573E5A3" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SolothurnSwitzerlandMember" xlink:label="loc_biib_SolothurnSwitzerlandMember_CFA37CC8889A51FF92756CB588DBB3E6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_FacilityLocationDomain_2E855BF3CCCC5CAFA6E0E2ACD573E5A3" xlink:to="loc_biib_SolothurnSwitzerlandMember_CFA37CC8889A51FF92756CB588DBB3E6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_6D1E30B9C9675E979902BBE3547B98F8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_ECCC302C81AF5278B02F9614EEA1E3ED" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_6D1E30B9C9675E979902BBE3547B98F8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConstructionInProgressGross" xlink:label="loc_us-gaap_ConstructionInProgressGross_E81946850E375125A3AB97BBFF0F4DF6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_ECCC302C81AF5278B02F9614EEA1E3ED" xlink:to="loc_us-gaap_ConstructionInProgressGross_E81946850E375125A3AB97BBFF0F4DF6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitment" xlink:label="loc_us-gaap_OtherCommitment_5AA0A6B7EA405B3C819FDAF92FA261FE" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_ECCC302C81AF5278B02F9614EEA1E3ED" xlink:to="loc_us-gaap_OtherCommitment_5AA0A6B7EA405B3C819FDAF92FA261FE" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RestructuringRestructuring" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RestructuringRestructuringDetailsTextual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_503FBD34F14BF2C42D247D4634EDCD1A" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_D1084D31E7A9495713A27D4634EC4308" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_503FBD34F14BF2C42D247D4634EDCD1A" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_D1084D31E7A9495713A27D4634EC4308" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestructuringPlanAxis" xlink:label="loc_us-gaap_RestructuringPlanAxis_1DC7077D69C9F3A0D84B7D4634EC41F7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_D1084D31E7A9495713A27D4634EC4308" xlink:to="loc_us-gaap_RestructuringPlanAxis_1DC7077D69C9F3A0D84B7D4634EC41F7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_6940280553D15B7670F77D4634EC9561_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_1DC7077D69C9F3A0D84B7D4634EC41F7" xlink:to="loc_us-gaap_RestructuringPlanDomain_6940280553D15B7670F77D4634EC9561_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_6940280553D15B7670F77D4634EC9561" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_1DC7077D69C9F3A0D84B7D4634EC41F7" xlink:to="loc_us-gaap_RestructuringPlanDomain_6940280553D15B7670F77D4634EC9561" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_A2017CorporateStrategyMember" xlink:label="loc_biib_A2017CorporateStrategyMember_42FA6363792A4F2B636D7D4634EC1C62" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_6940280553D15B7670F77D4634EC9561" xlink:to="loc_biib_A2017CorporateStrategyMember_42FA6363792A4F2B636D7D4634EC1C62" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_A8306F74F91E66A651AD7D4634ED8A76" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_503FBD34F14BF2C42D247D4634EDCD1A" xlink:to="loc_us-gaap_RestructuringCharges_A8306F74F91E66A651AD7D4634ED8A76" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RevenuesOtherRevenuesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_B348CE83198487DD378CA26AE8FB6ACF" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_683A7B374A5EC51C0FB9A26AE8F6CDE4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_B348CE83198487DD378CA26AE8FB6ACF" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_683A7B374A5EC51C0FB9A26AE8F6CDE4" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_2DB9D7BA3DAE67DE6545A26AE8F8B1CF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_683A7B374A5EC51C0FB9A26AE8F6CDE4" xlink:to="loc_us-gaap_TypeOfArrangementAxis_2DB9D7BA3DAE67DE6545A26AE8F8B1CF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_D28DF54F129A00DF2D37A26AE8F8F0A2_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_2DB9D7BA3DAE67DE6545A26AE8F8B1CF" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_D28DF54F129A00DF2D37A26AE8F8F0A2_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_D28DF54F129A00DF2D37A26AE8F8F0A2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_2DB9D7BA3DAE67DE6545A26AE8F8B1CF" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_D28DF54F129A00DF2D37A26AE8F8F0A2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CollaborativeArrangementMember" xlink:label="loc_us-gaap_CollaborativeArrangementMember_CE556ED3C64ADE07FDBDA26AE8F8E086" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_D28DF54F129A00DF2D37A26AE8F8F0A2" xlink:to="loc_us-gaap_CollaborativeArrangementMember_CE556ED3C64ADE07FDBDA26AE8F8E086" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_EAF8C9B528974580758FA26AE8F90955" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_683A7B374A5EC51C0FB9A26AE8F6CDE4" xlink:to="loc_srt_ProductOrServiceAxis_EAF8C9B528974580758FA26AE8F90955" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_785364464C95089F8702A26AE8F9F98A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_EAF8C9B528974580758FA26AE8F90955" xlink:to="loc_srt_ProductsAndServicesDomain_785364464C95089F8702A26AE8F9F98A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_785364464C95089F8702A26AE8F9F98A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_EAF8C9B528974580758FA26AE8F90955" xlink:to="loc_srt_ProductsAndServicesDomain_785364464C95089F8702A26AE8F9F98A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RoyaltyMember" xlink:label="loc_us-gaap_RoyaltyMember_91C155DBC7B7DD1B3E7BA26AE8F976E0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_785364464C95089F8702A26AE8F9F98A" xlink:to="loc_us-gaap_RoyaltyMember_91C155DBC7B7DD1B3E7BA26AE8F976E0" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_OthercorporaterevenuesMember" xlink:label="loc_biib_OthercorporaterevenuesMember_4FF0D635045391B346E6A26AE8FA1910" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_785364464C95089F8702A26AE8F9F98A" xlink:to="loc_biib_OthercorporaterevenuesMember_4FF0D635045391B346E6A26AE8FA1910" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProductAndServiceOtherMember" xlink:label="loc_us-gaap_ProductAndServiceOtherMember_207E1752E2C4DD45CD79A26AE8FAAB43" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_785364464C95089F8702A26AE8F9F98A" xlink:to="loc_us-gaap_ProductAndServiceOtherMember_207E1752E2C4DD45CD79A26AE8FAAB43" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MajorCustomersAxis" xlink:label="loc_srt_MajorCustomersAxis_3A9E5979823216E6E157A26AE8FA6874" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_683A7B374A5EC51C0FB9A26AE8F6CDE4" xlink:to="loc_srt_MajorCustomersAxis_3A9E5979823216E6E157A26AE8FA6874" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_BEDD9FE1BB9070BA6E8FA26AE8FA2C37_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_3A9E5979823216E6E157A26AE8FA6874" xlink:to="loc_srt_NameOfMajorCustomerDomain_BEDD9FE1BB9070BA6E8FA26AE8FA2C37_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_BEDD9FE1BB9070BA6E8FA26AE8FA2C37" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_3A9E5979823216E6E157A26AE8FA6874" xlink:to="loc_srt_NameOfMajorCustomerDomain_BEDD9FE1BB9070BA6E8FA26AE8FA2C37" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_AbbVieMember" xlink:label="loc_biib_AbbVieMember_B892AC0140A818B53965A26AE8FA8EED" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_BEDD9FE1BB9070BA6E8FA26AE8FA2C37" xlink:to="loc_biib_AbbVieMember_B892AC0140A818B53965A26AE8FA8EED" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SamsungBiosimilarAgreementMember" xlink:label="loc_biib_SamsungBiosimilarAgreementMember_EF2DC8A4432B2F67258FA26AE8FB53FA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_BEDD9FE1BB9070BA6E8FA26AE8FA2C37" xlink:to="loc_biib_SamsungBiosimilarAgreementMember_EF2DC8A4432B2F67258FA26AE8FB53FA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_4B4A5054BE9E917EAB29A26AE8FBFCCC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_B348CE83198487DD378CA26AE8FB6ACF" xlink:to="loc_us-gaap_Revenues_4B4A5054BE9E917EAB29A26AE8FBFCCC" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RevenuesReservesForDiscountsAndAllowancesDetails1" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ValuationAndQualifyingAccountsDisclosureLineItems" xlink:label="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_82F67BC524EF5D038B406A94BDDAA545" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ValuationAndQualifyingAccountsDisclosureTable" xlink:label="loc_srt_ValuationAndQualifyingAccountsDisclosureTable_EF0C6E91B9FE5BAD9803E37FED4EF4F1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_82F67BC524EF5D038B406A94BDDAA545" xlink:to="loc_srt_ValuationAndQualifyingAccountsDisclosureTable_EF0C6E91B9FE5BAD9803E37FED4EF4F1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ValuationAllowancesAndReservesTypeAxis" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_5430ADC56DF65CB482EAECAC347452B0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureTable_EF0C6E91B9FE5BAD9803E37FED4EF4F1" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_5430ADC56DF65CB482EAECAC347452B0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ValuationAllowancesAndReservesDomain" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_987EDE83BFA0424BBBF93D2B6B684322_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_5430ADC56DF65CB482EAECAC347452B0" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_987EDE83BFA0424BBBF93D2B6B684322_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ValuationAllowancesAndReservesDomain" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_677861BB51255B2FBEA6DDD30EBA6AD7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_5430ADC56DF65CB482EAECAC347452B0" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_677861BB51255B2FBEA6DDD30EBA6AD7" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ReserveforCashDiscountsMember" xlink:label="loc_biib_ReserveforCashDiscountsMember_963E387120E7593FBF909E73296FDB03" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_677861BB51255B2FBEA6DDD30EBA6AD7" xlink:to="loc_biib_ReserveforCashDiscountsMember_963E387120E7593FBF909E73296FDB03" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ContractualAdjustmentsMember" xlink:label="loc_biib_ContractualAdjustmentsMember_4B3BDC40910455B38ECB2448AB81BD14" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_677861BB51255B2FBEA6DDD30EBA6AD7" xlink:to="loc_biib_ContractualAdjustmentsMember_4B3BDC40910455B38ECB2448AB81BD14" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SalesReturnsAndAllowancesMember" xlink:label="loc_us-gaap_SalesReturnsAndAllowancesMember_D85748124B7B5D44AA81BB20ECA768C1" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_677861BB51255B2FBEA6DDD30EBA6AD7" xlink:to="loc_us-gaap_SalesReturnsAndAllowancesMember_D85748124B7B5D44AA81BB20ECA768C1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ValuationAllowancesAndReservesBalance" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesBalance_90245718DA7A5EB9B1FE84F03BA4C36E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_82F67BC524EF5D038B406A94BDDAA545" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesBalance_90245718DA7A5EB9B1FE84F03BA4C36E" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear" xlink:label="loc_biib_CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear_66CC6BA826495FA3B7FFA24626C069C9" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_82F67BC524EF5D038B406A94BDDAA545" xlink:to="loc_biib_CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear_66CC6BA826495FA3B7FFA24626C069C9" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears" xlink:label="loc_biib_AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears_31BC3F07315459B8A88D391001B0BEE5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_82F67BC524EF5D038B406A94BDDAA545" xlink:to="loc_biib_AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears_31BC3F07315459B8A88D391001B0BEE5" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear" xlink:label="loc_biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear_603DB80E077B508FA53E6FD32789A2DA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_82F67BC524EF5D038B406A94BDDAA545" xlink:to="loc_biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear_603DB80E077B508FA53E6FD32789A2DA" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear" xlink:label="loc_biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear_F8417F93264B586AAF05F8FD79337D0C" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ValuationAndQualifyingAccountsDisclosureLineItems_82F67BC524EF5D038B406A94BDDAA545" xlink:to="loc_biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear_F8417F93264B586AAF05F8FD79337D0C" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RevenuesReservesForDiscountsAndAllowancesDetails2" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_F5564C33D08B594E9C5F70893973457D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_FAD450A9144C5C1DB7454A2FA4870A17" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_F5564C33D08B594E9C5F70893973457D" xlink:to="loc_us-gaap_StatementTable_FAD450A9144C5C1DB7454A2FA4870A17" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_84E2133FC9165F21B2A06980D49EAF09" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_FAD450A9144C5C1DB7454A2FA4870A17" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_84E2133FC9165F21B2A06980D49EAF09" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_3FE99CF44EEB101C7FB13CAA464FB26E_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_84E2133FC9165F21B2A06980D49EAF09" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_3FE99CF44EEB101C7FB13CAA464FB26E_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_03A10570EA455935B150FBB631F44F5D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_84E2133FC9165F21B2A06980D49EAF09" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_03A10570EA455935B150FBB631F44F5D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountsReceivableMember" xlink:label="loc_us-gaap_AccountsReceivableMember_0A197196BEB65579906FABD8B005A614" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_03A10570EA455935B150FBB631F44F5D" xlink:to="loc_us-gaap_AccountsReceivableMember_0A197196BEB65579906FABD8B005A614" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCurrentLiabilitiesMember" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_0C79EDCAB97754B2B18718BB94D38A7E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_03A10570EA455935B150FBB631F44F5D" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_0C79EDCAB97754B2B18718BB94D38A7E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ValuationAllowancesAndReservesBalance" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesBalance_B1B293B31D6453038CAA31CF714D4E19" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_F5564C33D08B594E9C5F70893973457D" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesBalance_B1B293B31D6453038CAA31CF714D4E19" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RevenuesRevenues" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RevenuesRevenuesByProductDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_23FD8974ADF459258597B02F0571D968" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_FEFFA86587015DB9B986A16E9E1F02D8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_23FD8974ADF459258597B02F0571D968" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_FEFFA86587015DB9B986A16E9E1F02D8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_27895F372FED5B10AF7DAA0B877133EA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_FEFFA86587015DB9B986A16E9E1F02D8" xlink:to="loc_srt_ProductOrServiceAxis_27895F372FED5B10AF7DAA0B877133EA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_8613C20FE67FFC81C5E0C1F00F23298D_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_27895F372FED5B10AF7DAA0B877133EA" xlink:to="loc_srt_ProductsAndServicesDomain_8613C20FE67FFC81C5E0C1F00F23298D_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_27895F372FED5B10AF7DAA0B877133EA" xlink:to="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TecfideraMember" xlink:label="loc_biib_TecfideraMember_5AF137B8D58251068702ED68B6C98484" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_TecfideraMember_5AF137B8D58251068702ED68B6C98484" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_InterferonMember" xlink:label="loc_biib_InterferonMember_A1D036EC49AE52C3A6DDF92D5FD90ED8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_InterferonMember_A1D036EC49AE52C3A6DDF92D5FD90ED8" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TysabriProductMember" xlink:label="loc_biib_TysabriProductMember_6A6DBEF2B4675F68A60500F1A2B7679A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_TysabriProductMember_6A6DBEF2B4675F68A60500F1A2B7679A" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FAMPYRAMember" xlink:label="loc_biib_FAMPYRAMember_B80B9C9E3AA95300B0589AE97A40627B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_FAMPYRAMember_B80B9C9E3AA95300B0589AE97A40627B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ZINBRYTAMember" xlink:label="loc_biib_ZINBRYTAMember_6CA5119950195F73B20544D642B83F87" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_ZINBRYTAMember_6CA5119950195F73B20544D642B83F87" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SPINRAZAMember" xlink:label="loc_biib_SPINRAZAMember_DE462FFC7BED530D9412EE733CADD3AD" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_SPINRAZAMember_DE462FFC7BED530D9412EE733CADD3AD" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ELOCTATEMember" xlink:label="loc_biib_ELOCTATEMember_ED32A0D8AB305C60A1CA04D49402E248" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_ELOCTATEMember_ED32A0D8AB305C60A1CA04D49402E248" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ALPROLIXMember" xlink:label="loc_biib_ALPROLIXMember_E81B8B73E20753E2A94D2CF7A859438C" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_ALPROLIXMember_E81B8B73E20753E2A94D2CF7A859438C" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FUMADERMMember" xlink:label="loc_biib_FUMADERMMember_ABA77CD88E1B58B883D9F09E73E3AC19" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_FUMADERMMember_ABA77CD88E1B58B883D9F09E73E3AC19" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_BENEPALIMember" xlink:label="loc_biib_BENEPALIMember_05A0894971445778B41995E0FCE5FCCE" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_BENEPALIMember_05A0894971445778B41995E0FCE5FCCE" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_FLIXABIMember" xlink:label="loc_biib_FLIXABIMember_311663491BB650EBBB43A1510A99B1AD" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_biib_FLIXABIMember_311663491BB650EBBB43A1510A99B1AD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProductMember" xlink:label="loc_us-gaap_ProductMember_682D6CF87DA35ACAB0A5469A26223DB4" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_A2C49173388955DB8AABACF2367D749F" xlink:to="loc_us-gaap_ProductMember_682D6CF87DA35ACAB0A5469A26223DB4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_ABA757937D0F573CBE46B6784534CEE0" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_FEFFA86587015DB9B986A16E9E1F02D8" xlink:to="loc_srt_StatementGeographicalAxis_ABA757937D0F573CBE46B6784534CEE0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_A24B3563ADD09655E301C1F00F237125_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_ABA757937D0F573CBE46B6784534CEE0" xlink:to="loc_srt_SegmentGeographicalDomain_A24B3563ADD09655E301C1F00F237125_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_E7C591685C8D5ADF861C5C94D5B1C297" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_ABA757937D0F573CBE46B6784534CEE0" xlink:to="loc_srt_SegmentGeographicalDomain_E7C591685C8D5ADF861C5C94D5B1C297" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_US" xlink:label="loc_country_US_AADB6AF7F5895AA5A8CFAA10AD50E64A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_E7C591685C8D5ADF861C5C94D5B1C297" xlink:to="loc_country_US_AADB6AF7F5895AA5A8CFAA10AD50E64A" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_RestOfWorldMember" xlink:label="loc_biib_RestOfWorldMember_5A83604FA9C2535FBD43DA21B6FE97E6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_E7C591685C8D5ADF861C5C94D5B1C297" xlink:to="loc_biib_RestOfWorldMember_5A83604FA9C2535FBD43DA21B6FE97E6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_0B38C48F3C2D5D20A063D865AD972565" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_23FD8974ADF459258597B02F0571D968" xlink:to="loc_us-gaap_Revenues_0B38C48F3C2D5D20A063D865AD972565" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RevenuesRevenuesDetailsTextual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_EFD9CF02F3895D94A182BE303BC4EC0A" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_A47F801538115345B15CDCC3B1103E9F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_EFD9CF02F3895D94A182BE303BC4EC0A" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_A47F801538115345B15CDCC3B1103E9F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MajorCustomersAxis" xlink:label="loc_srt_MajorCustomersAxis_8703AEB90EE256819EAF326C397F4BB7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_A47F801538115345B15CDCC3B1103E9F" xlink:to="loc_srt_MajorCustomersAxis_8703AEB90EE256819EAF326C397F4BB7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_580E9AD2D1FD23BD502F3C312D0467AC_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_8703AEB90EE256819EAF326C397F4BB7" xlink:to="loc_srt_NameOfMajorCustomerDomain_580E9AD2D1FD23BD502F3C312D0467AC_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_7D2C2EC93FA15BCA92DD7CA6C19C1D96" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_8703AEB90EE256819EAF326C397F4BB7" xlink:to="loc_srt_NameOfMajorCustomerDomain_7D2C2EC93FA15BCA92DD7CA6C19C1D96" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DistributorOneMember" xlink:label="loc_biib_DistributorOneMember_80C182710EAC5E90B6B4172AC8B5A3E7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_7D2C2EC93FA15BCA92DD7CA6C19C1D96" xlink:to="loc_biib_DistributorOneMember_80C182710EAC5E90B6B4172AC8B5A3E7" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_DistributorTwoMember" xlink:label="loc_biib_DistributorTwoMember_0BDD35D81F9453688FFB3B47B3E8E1E8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_7D2C2EC93FA15BCA92DD7CA6C19C1D96" xlink:to="loc_biib_DistributorTwoMember_0BDD35D81F9453688FFB3B47B3E8E1E8" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_EntityWidePercentageOfRevenueFromMajorDistributors" xlink:label="loc_biib_EntityWidePercentageOfRevenueFromMajorDistributors_174B50C8ABD15242B038E18647AF9B4F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_EFD9CF02F3895D94A182BE303BC4EC0A" xlink:to="loc_biib_EntityWidePercentageOfRevenueFromMajorDistributors_174B50C8ABD15242B038E18647AF9B4F" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RevenuesRevenuesFromAntiCd20TherapeuticProgramsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_939782155DD8D585683E9878FF6A70CC" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_339377CEDA51BE8E33689878FF68C04F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_939782155DD8D585683E9878FF6A70CC" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_339377CEDA51BE8E33689878FF68C04F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_8477496163EEFEDC497A9878FF699E1C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_339377CEDA51BE8E33689878FF68C04F" xlink:to="loc_srt_ProductOrServiceAxis_8477496163EEFEDC497A9878FF699E1C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_2B2CE2F847E1E1DA66479878FF697691_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_8477496163EEFEDC497A9878FF699E1C" xlink:to="loc_srt_ProductsAndServicesDomain_2B2CE2F847E1E1DA66479878FF697691_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_2B2CE2F847E1E1DA66479878FF697691" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_8477496163EEFEDC497A9878FF699E1C" xlink:to="loc_srt_ProductsAndServicesDomain_2B2CE2F847E1E1DA66479878FF697691" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_Revenuesfromanticd20therapeuticprogramsMember" xlink:label="loc_biib_Revenuesfromanticd20therapeuticprogramsMember_32A094AD310485406CF79878FF69F563" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_2B2CE2F847E1E1DA66479878FF697691" xlink:to="loc_biib_Revenuesfromanticd20therapeuticprogramsMember_32A094AD310485406CF79878FF69F563" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MajorCustomersAxis" xlink:label="loc_srt_MajorCustomersAxis_8CB475938871BE68B07F9878FF6925FB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_339377CEDA51BE8E33689878FF68C04F" xlink:to="loc_srt_MajorCustomersAxis_8CB475938871BE68B07F9878FF6925FB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_7D9964005D9F96A052949878FF6AE24D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_8CB475938871BE68B07F9878FF6925FB" xlink:to="loc_srt_NameOfMajorCustomerDomain_7D9964005D9F96A052949878FF6AE24D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_7D9964005D9F96A052949878FF6AE24D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_8CB475938871BE68B07F9878FF6925FB" xlink:to="loc_srt_NameOfMajorCustomerDomain_7D9964005D9F96A052949878FF6AE24D" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_RocheGroupGenentechMember" xlink:label="loc_biib_RocheGroupGenentechMember_314CA60460E8477AD2169878FF6A851F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_7D9964005D9F96A052949878FF6AE24D" xlink:to="loc_biib_RocheGroupGenentechMember_314CA60460E8477AD2169878FF6A851F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ShareOfCoPromotionProfits" xlink:label="loc_biib_ShareOfCoPromotionProfits_71AECA4485EFC29B778D9878FF6AE29E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_939782155DD8D585683E9878FF6A70CC" xlink:to="loc_biib_ShareOfCoPromotionProfits_71AECA4485EFC29B778D9878FF6AE29E" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_OtherrevenuesfromantiCD20therapeuticprograms" xlink:label="loc_biib_OtherrevenuesfromantiCD20therapeuticprograms_37EABB386A0AB71D792E9878FF6BB676" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_939782155DD8D585683E9878FF6A70CC" xlink:to="loc_biib_OtherrevenuesfromantiCD20therapeuticprograms_37EABB386A0AB71D792E9878FF6BB676" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_604E513E4BE408DBC2559878FF6B4A7B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_939782155DD8D585683E9878FF6A70CC" xlink:to="loc_us-gaap_Revenues_604E513E4BE408DBC2559878FF6B4A7B" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/RevenuesRevenuesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPayments" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_6916E6958263CD5161F7A26C60E2AF8E" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_8316FB653060C6B34DBFA26C60E092A3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_6916E6958263CD5161F7A26C60E2AF8E" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_8316FB653060C6B34DBFA26C60E092A3" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_8FC1C747F71333C1FCF4A26C60E1F79A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_8316FB653060C6B34DBFA26C60E092A3" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_8FC1C747F71333C1FCF4A26C60E1F79A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_5B39AA5016C0C594F420A26C60E1F2AD_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_8FC1C747F71333C1FCF4A26C60E1F79A" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_5B39AA5016C0C594F420A26C60E1F2AD_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_5B39AA5016C0C594F420A26C60E1F2AD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_8FC1C747F71333C1FCF4A26C60E1F79A" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_5B39AA5016C0C594F420A26C60E1F2AD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_62A2060D3DD122A45A64A26C60E128A1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_5B39AA5016C0C594F420A26C60E1F2AD" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_62A2060D3DD122A45A64A26C60E128A1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_E5661411C07BE39B8D7EA26C60E15EF3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_5B39AA5016C0C594F420A26C60E1F2AD" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_E5661411C07BE39B8D7EA26C60E15EF3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ParentMember" xlink:label="loc_us-gaap_ParentMember_08803C9C93E9232E1119A26C60E262D0" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_5B39AA5016C0C594F420A26C60E1F2AD" xlink:to="loc_us-gaap_ParentMember_08803C9C93E9232E1119A26C60E262D0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_517FBC37B215CDEBCC53A26C60E29D67" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_6916E6958263CD5161F7A26C60E2AF8E" xlink:to="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_517FBC37B215CDEBCC53A26C60E29D67" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_7737A516CBCFEB7A183CA26C60E2205B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_517FBC37B215CDEBCC53A26C60E29D67" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_7737A516CBCFEB7A183CA26C60E2205B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_F1193052F8C49EB05944A26C60E36B20" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_517FBC37B215CDEBCC53A26C60E29D67" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_F1193052F8C49EB05944A26C60E36B20" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_671B291F18086AFB3BDDA26C60E3A61D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_517FBC37B215CDEBCC53A26C60E29D67" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_671B291F18086AFB3BDDA26C60E3A61D" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses" xlink:label="loc_biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses_4D7116CD3BB8D26CF570A26C60E300AC" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_517FBC37B215CDEBCC53A26C60E29D67" xlink:to="loc_biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses_4D7116CD3BB8D26CF570A26C60E300AC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_120ED1CFCA6309AA2975A26C60E87687" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_517FBC37B215CDEBCC53A26C60E29D67" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_120ED1CFCA6309AA2975A26C60E87687" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails1" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_3271F89AB27050C48B9FE4CFF872EBE0" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_6029BED85ED3556FB6D6EA739FECCC36" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_3271F89AB27050C48B9FE4CFF872EBE0" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_6029BED85ED3556FB6D6EA739FECCC36" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_9D438A63654A59DA874B433B40BBD649" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_6029BED85ED3556FB6D6EA739FECCC36" xlink:to="loc_us-gaap_AwardTypeAxis_9D438A63654A59DA874B433B40BBD649" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8C882B2FC00179EA46A93C312BC7C78A_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_9D438A63654A59DA874B433B40BBD649" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8C882B2FC00179EA46A93C312BC7C78A_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_9D438A63654A59DA874B433B40BBD649" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_MarketStockUnitsMember" xlink:label="loc_biib_MarketStockUnitsMember_73722F2231FD5AFBB0DC343A6559EF39" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:to="loc_biib_MarketStockUnitsMember_73722F2231FD5AFBB0DC343A6559EF39" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_TimeVestedRestrictedStockUnitsMember" xlink:label="loc_biib_TimeVestedRestrictedStockUnitsMember_290F1AA819B0517896CB6EA57734E1ED" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:to="loc_biib_TimeVestedRestrictedStockUnitsMember_290F1AA819B0517896CB6EA57734E1ED" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_CashSettledPerformanceSharesMember" xlink:label="loc_biib_CashSettledPerformanceSharesMember_2F07DB0C399556CA93D3A0BFC0C74741" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:to="loc_biib_CashSettledPerformanceSharesMember_2F07DB0C399556CA93D3A0BFC0C74741" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PerformanceSharesMember" xlink:label="loc_us-gaap_PerformanceSharesMember_E38F84EADA8B5D178BDB85B733356759" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:to="loc_us-gaap_PerformanceSharesMember_E38F84EADA8B5D178BDB85B733356759" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PerformanceStockUnitsSettledinStockMember" xlink:label="loc_biib_PerformanceStockUnitsSettledinStockMember_D2BC8A8C5F2D5A8CB813FE6AA59D16A4" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:to="loc_biib_PerformanceStockUnitsSettledinStockMember_D2BC8A8C5F2D5A8CB813FE6AA59D16A4" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PerformanceStockUnitsSettledinCashMember" xlink:label="loc_biib_PerformanceStockUnitsSettledinCashMember_884975920AD9572C893329E2492ABE0B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:to="loc_biib_PerformanceStockUnitsSettledinCashMember_884975920AD9572C893329E2492ABE0B" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_EmployeeStockPurchasePlanMember" xlink:label="loc_biib_EmployeeStockPurchasePlanMember_D076F65636385BE7AED866D7DC39EEA6" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_B4C2084C56565AE193DF37F9B216DFDB" xlink:to="loc_biib_EmployeeStockPurchasePlanMember_D076F65636385BE7AED866D7DC39EEA6" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract" xlink:label="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_48516D7753CA5D5BB63F346D065AF9FE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_3271F89AB27050C48B9FE4CFF872EBE0" xlink:to="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_48516D7753CA5D5BB63F346D065AF9FE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_574018B8CD4D5949866335F43CC37C14" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_48516D7753CA5D5BB63F346D065AF9FE" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_574018B8CD4D5949866335F43CC37C14" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_6478D4D7B63B55AD92E569BCABC0F7AF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_48516D7753CA5D5BB63F346D065AF9FE" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_6478D4D7B63B55AD92E569BCABC0F7AF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_8C6AE64B87475634B53575636E9ECA69" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_48516D7753CA5D5BB63F346D065AF9FE" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_8C6AE64B87475634B53575636E9ECA69" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses" xlink:label="loc_biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses_5B5280AB85B0589484AF8C7D3482C44C" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_48516D7753CA5D5BB63F346D065AF9FE" xlink:to="loc_biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses_5B5280AB85B0589484AF8C7D3482C44C" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsShareBasedPaymentsDetailsTextual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_BC5B9825DB3C54619EA667E0CA9C9B97" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_828DD4ACAE045E5FBEB3ABCFC09D9F8C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_BC5B9825DB3C54619EA667E0CA9C9B97" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_828DD4ACAE045E5FBEB3ABCFC09D9F8C" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_8AD385DD8A56584FAEED2851EE2C226E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_828DD4ACAE045E5FBEB3ABCFC09D9F8C" xlink:to="loc_us-gaap_AwardTypeAxis_8AD385DD8A56584FAEED2851EE2C226E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_79AB0EF9E5BBBA3B82003C312D336F3E_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_8AD385DD8A56584FAEED2851EE2C226E" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_79AB0EF9E5BBBA3B82003C312D336F3E_7752473BA0695B63AEDA33E026DE3EDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_32E7925014CA50CEB5C6E1A8347E232F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_8AD385DD8A56584FAEED2851EE2C226E" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_32E7925014CA50CEB5C6E1A8347E232F" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PerformanceStockUnitsSettledinStockMember" xlink:label="loc_biib_PerformanceStockUnitsSettledinStockMember_459B13490D735F1B9F6249153F66E2F2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_32E7925014CA50CEB5C6E1A8347E232F" xlink:to="loc_biib_PerformanceStockUnitsSettledinStockMember_459B13490D735F1B9F6249153F66E2F2" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_PerformanceStockUnitsSettledinCashMember" xlink:label="loc_biib_PerformanceStockUnitsSettledinCashMember_F059A423E39A57BAB8BFAADFF78200C4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_32E7925014CA50CEB5C6E1A8347E232F" xlink:to="loc_biib_PerformanceStockUnitsSettledinCashMember_F059A423E39A57BAB8BFAADFF78200C4" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent" xlink:label="loc_biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent_F240E4C7A1CE5B4F9E0F8B71C0F3EB72" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_BC5B9825DB3C54619EA667E0CA9C9B97" xlink:to="loc_biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent_F240E4C7A1CE5B4F9E0F8B71C0F3EB72" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent" xlink:label="loc_biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent_2E2214BF413754EC90EE1E09EB38AB33" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_BC5B9825DB3C54619EA667E0CA9C9B97" xlink:to="loc_biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent_2E2214BF413754EC90EE1E09EB38AB33" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_3A11673622E4574087030C40709B1036" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_BC5B9825DB3C54619EA667E0CA9C9B97" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_3A11673622E4574087030C40709B1036" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_4D86CC10323256B7859526E47BF43F04" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_BC5B9825DB3C54619EA667E0CA9C9B97" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_4D86CC10323256B7859526E47BF43F04" xlink:type="arc" />
    <link:loc xlink:href="biib-20180930.xsd#biib_NumberOfDaysForCalculationOfAverageClosingStockPrice" xlink:label="loc_biib_NumberOfDaysForCalculationOfAverageClosingStockPrice_A7914543C3CE518482C2A7EDC7145889" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_BC5B9825DB3C54619EA667E0CA9C9B97" xlink:to="loc_biib_NumberOfDaysForCalculationOfAverageClosingStockPrice_A7914543C3CE518482C2A7EDC7145889" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPolicies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_8F44556B56C4AB05FA289878FFA5792A" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_770BB1B906864503203C9878FFA1259B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_8F44556B56C4AB05FA289878FFA5792A" xlink:to="loc_us-gaap_StatementTable_770BB1B906864503203C9878FFA1259B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2474623287ED81875A0B9878FFA28908" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_770BB1B906864503203C9878FFA1259B" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2474623287ED81875A0B9878FFA28908" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_9A2CDF2F94FD30BFF3349878FFA27CCD_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2474623287ED81875A0B9878FFA28908" xlink:to="loc_us-gaap_TypeOfAdoptionMember_9A2CDF2F94FD30BFF3349878FFA27CCD_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_9A2CDF2F94FD30BFF3349878FFA27CCD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2474623287ED81875A0B9878FFA28908" xlink:to="loc_us-gaap_TypeOfAdoptionMember_9A2CDF2F94FD30BFF3349878FFA27CCD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountingStandardsUpdate201601Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201601Member_0DD1FC0C9A988EBA027A9878FFA2F253" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_9A2CDF2F94FD30BFF3349878FFA27CCD" xlink:to="loc_us-gaap_AccountingStandardsUpdate201601Member_0DD1FC0C9A988EBA027A9878FFA2F253" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountingStandardsUpdate201616Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201616Member_AF36066533787353F28A9878FFA22337" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_9A2CDF2F94FD30BFF3349878FFA27CCD" xlink:to="loc_us-gaap_AccountingStandardsUpdate201616Member_AF36066533787353F28A9878FFA22337" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountingStandardsUpdate201708Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201708Member_FF739E4D75A70C078B439887DB9BDADB" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_9A2CDF2F94FD30BFF3349878FFA27CCD" xlink:to="loc_us-gaap_AccountingStandardsUpdate201708Member_FF739E4D75A70C078B439887DB9BDADB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_4031D5AD656208F4745D9878FFA36D15" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_770BB1B906864503203C9878FFA1259B" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_4031D5AD656208F4745D9878FFA36D15" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_4B724D8B35EE65079EE59878FFA3F83A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_4031D5AD656208F4745D9878FFA36D15" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_4B724D8B35EE65079EE59878FFA3F83A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_4B724D8B35EE65079EE59878FFA3F83A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_4031D5AD656208F4745D9878FFA36D15" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_4B724D8B35EE65079EE59878FFA3F83A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpenseMember" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpenseMember_8B9D7ECE3DA4354AF8CC9878FFA4B018" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_4B724D8B35EE65079EE59878FFA3F83A" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpenseMember_8B9D7ECE3DA4354AF8CC9878FFA4B018" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_C82CA8B867520613D7F59878FFA4D948" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_770BB1B906864503203C9878FFA1259B" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_C82CA8B867520613D7F59878FFA4D948" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_F54DB0C394831EAE86E69878FFA44178_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_C82CA8B867520613D7F59878FFA4D948" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_F54DB0C394831EAE86E69878FFA44178_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_F54DB0C394831EAE86E69878FFA44178" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_C82CA8B867520613D7F59878FFA4D948" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_F54DB0C394831EAE86E69878FFA44178" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_11664C3A9104ED062D1E9878FFA462BD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_F54DB0C394831EAE86E69878FFA44178" xlink:to="loc_us-gaap_RetainedEarningsMember_11664C3A9104ED062D1E9878FFA462BD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredIncomeTaxChargesMember" xlink:label="loc_us-gaap_DeferredIncomeTaxChargesMember_A1F0291E4522FED26E749878FFA55D81" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_F54DB0C394831EAE86E69878FFA44178" xlink:to="loc_us-gaap_DeferredIncomeTaxChargesMember_A1F0291E4522FED26E749878FFA55D81" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_62B25ABA757ECE9F454B9878FFA5EA17" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8F44556B56C4AB05FA289878FFA5792A" xlink:to="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_62B25ABA757ECE9F454B9878FFA5EA17" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PriorPeriodReclassificationAdjustment" xlink:label="loc_us-gaap_PriorPeriodReclassificationAdjustment_21BD24CCCE42BAFD207E9878FFA645DB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8F44556B56C4AB05FA289878FFA5792A" xlink:to="loc_us-gaap_PriorPeriodReclassificationAdjustment_21BD24CCCE42BAFD207E9878FFA645DB" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesPolicies" xlink:type="extended" />
</link:linkbase>
