

<!--XBRL Document Created with WebFilings-->
<!-- -->
<xsd:schema attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.siga.com/20131231" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:nonnum="http://www.xbrl.org/dtr/type/non-numeric" xmlns:num="http://www.xbrl.org/dtr/type/numeric" xmlns:siga="http://www.siga.com/20131231" xmlns:us-types="http://fasb.org/us-types/2013-01-31" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <xsd:import namespace="http://fasb.org/us-gaap/2013-01-31" schemaLocation="http://xbrl.fasb.org/us-gaap/2013/elts/us-gaap-2013-01-31.xsd" />
  <xsd:import namespace="http://fasb.org/us-roles/2013-01-31" schemaLocation="http://xbrl.fasb.org/us-gaap/2013/elts/us-roles-2013-01-31.xsd" />
  <xsd:import namespace="http://fasb.org/us-types/2013-01-31" schemaLocation="http://xbrl.fasb.org/us-gaap/2013/elts/us-types-2013-01-31.xsd" />
  <xsd:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" />
  <xsd:import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" />
  <xsd:import namespace="http://www.xbrl.org/2009/arcrole/fact-explanatoryFact" schemaLocation="http://www.xbrl.org/lrr/arcrole/factExplanatory-2009-12-16.xsd" />
  <xsd:import namespace="http://www.xbrl.org/2009/role/negated" schemaLocation="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd" />
  <xsd:import namespace="http://www.xbrl.org/2009/role/net" schemaLocation="http://www.xbrl.org/lrr/role/net-2009-12-16.xsd" />
  <xsd:import namespace="http://www.xbrl.org/dtr/type/non-numeric" schemaLocation="http://www.xbrl.org/dtr/type/nonNumeric-2009-12-16.xsd" />
  <xsd:import namespace="http://www.xbrl.org/dtr/type/numeric" schemaLocation="http://www.xbrl.org/dtr/type/numeric-2009-12-16.xsd" />
  <xsd:import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/country/2013-01-31" schemaLocation="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/currency/2012-01-31" schemaLocation="http://xbrl.sec.gov/currency/2012/currency-2012-01-31.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/dei/2013-01-31" schemaLocation="http://xbrl.sec.gov/dei/2013/dei-2013-01-31.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/exch/2013-01-31" schemaLocation="http://xbrl.sec.gov/exch/2013/exch-2013-01-31.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/invest/2013-01-31" schemaLocation="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/naics/2011-01-31" schemaLocation="http://xbrl.sec.gov/naics/2011/naics-2011-01-31.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/sic/2011-01-31" schemaLocation="http://xbrl.sec.gov/sic/2011/sic-2011-01-31.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/stpr/2011-01-31" schemaLocation="http://xbrl.sec.gov/stpr/2011/stpr-2011-01-31.xsd" />
  <xsd:annotation>
    <xsd:appinfo>
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="siga-20131231_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="siga-20131231_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="siga-20131231_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="siga-20131231_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:type="simple" />
      <link:roleType id="AccruedExpensesAndOtherCurrentLiabilities" roleURI="http://www.siga.com/role/AccruedExpensesAndOtherCurrentLiabilities">
        <link:definition>2116100 - Disclosure - Accrued Expenses and Other Current Liabilities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesAndOtherCurrentLiabilitiesDetails" roleURI="http://www.siga.com/role/AccruedExpensesAndOtherCurrentLiabilitiesDetails">
        <link:definition>2416402 - Disclosure - Accrued Expenses and Other Current Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesAndOtherCurrentLiabilitiesTables" roleURI="http://www.siga.com/role/AccruedExpensesAndOtherCurrentLiabilitiesTables">
        <link:definition>2316301 - Disclosure - Accrued Expenses and Other Current Liabilities (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsAndContingencies" roleURI="http://www.siga.com/role/CommitmentsAndContingencies">
        <link:definition>2119100 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsAndContingenciesDetails" roleURI="http://www.siga.com/role/CommitmentsAndContingenciesDetails">
        <link:definition>2419402 - Disclosure - Commitments and Contingencies (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsAndContingenciesOperatingLeaseCommitmentsTables" roleURI="http://www.siga.com/role/CommitmentsAndContingenciesOperatingLeaseCommitmentsTables">
        <link:definition>2319301 - Disclosure - Commitments and Contingencies Operating Lease Commitments (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsAndContingenciesOperatingLeasesDetails" roleURI="http://www.siga.com/role/CommitmentsAndContingenciesOperatingLeasesDetails">
        <link:definition>2419403 - Disclosure - Commitments and Contingencies Operating Leases (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedBalanceSheets" roleURI="http://www.siga.com/role/ConsolidatedBalanceSheets">
        <link:definition>1001000 - Statement - CONSOLIDATED BALANCE SHEETS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedBalanceSheetsParentheticals" roleURI="http://www.siga.com/role/ConsolidatedBalanceSheetsParentheticals">
        <link:definition>1001501 - Statement - CONSOLIDATED BALANCE SHEETS (Parentheticals)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsOfCashFlows" roleURI="http://www.siga.com/role/ConsolidatedStatementsOfCashFlows">
        <link:definition>1004000 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsOfChangesInStockholdersEquity" roleURI="http://www.siga.com/role/ConsolidatedStatementsOfChangesInStockholdersEquity">
        <link:definition>1003000 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsOfOperationsAndComprehensiveIncomeLoss" roleURI="http://www.siga.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveIncomeLoss">
        <link:definition>1002000 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME/LOSS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Debt" roleURI="http://www.siga.com/role/Debt">
        <link:definition>2111100 - Disclosure - Debt</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DebtDetails" roleURI="http://www.siga.com/role/DebtDetails">
        <link:definition>2411402 - Disclosure - Debt (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DebtTables" roleURI="http://www.siga.com/role/DebtTables">
        <link:definition>2311301 - Disclosure - Debt (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DocumentAndEntityInformationDocument" roleURI="http://www.siga.com/role/DocumentAndEntityInformationDocument">
        <link:definition>0001000 - Document - Document and Entity Information Document</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInformationByQuarterUnaudited" roleURI="http://www.siga.com/role/FinancialInformationByQuarterUnaudited">
        <link:definition>2120100 - Disclosure - Financial Information by Quarter (Unaudited)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInformationByQuarterUnauditedDetails" roleURI="http://www.siga.com/role/FinancialInformationByQuarterUnauditedDetails">
        <link:definition>2420402 - Disclosure - Financial Information by Quarter (Unaudited) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FinancialInformationByQuarterUnauditedTables" roleURI="http://www.siga.com/role/FinancialInformationByQuarterUnauditedTables">
        <link:definition>2320301 - Disclosure - Financial Information by Quarter (Unaudited) (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxes" roleURI="http://www.siga.com/role/IncomeTaxes">
        <link:definition>2117100 - Disclosure - Income Taxes</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesDetails" roleURI="http://www.siga.com/role/IncomeTaxesDetails">
        <link:definition>2417402 - Disclosure - Income Taxes (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesTables" roleURI="http://www.siga.com/role/IncomeTaxesTables">
        <link:definition>2317301 - Disclosure - Income Taxes (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Inventory" roleURI="http://www.siga.com/role/Inventory">
        <link:definition>2113100 - Disclosure - Inventory</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InventoryDetails" roleURI="http://www.siga.com/role/InventoryDetails">
        <link:definition>2413402 - Disclosure - Inventory (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InventoryInventoryTables" roleURI="http://www.siga.com/role/InventoryInventoryTables">
        <link:definition>2313301 - Disclosure - Inventory Inventory (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="OrganizationAndBasisOfPresentation" roleURI="http://www.siga.com/role/OrganizationAndBasisOfPresentation">
        <link:definition>2101100 - Disclosure - Organization and Basis of Presentation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ProcurementContractAndResearchAgreements" roleURI="http://www.siga.com/role/ProcurementContractAndResearchAgreements">
        <link:definition>2105100 - Disclosure - Procurement Contract and Research Agreements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ProcurementContractAndResearchAgreementsDetails" roleURI="http://www.siga.com/role/ProcurementContractAndResearchAgreementsDetails">
        <link:definition>2405401 - Disclosure - Procurement Contract and Research Agreements (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyPlantAndEquipment" roleURI="http://www.siga.com/role/PropertyPlantAndEquipment">
        <link:definition>2114100 - Disclosure - Property, Plant and Equipment</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyPlantAndEquipmentDetails" roleURI="http://www.siga.com/role/PropertyPlantAndEquipmentDetails">
        <link:definition>2414402 - Disclosure - Property, Plant and Equipment (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyPlantAndEquipmentTables" roleURI="http://www.siga.com/role/PropertyPlantAndEquipmentTables">
        <link:definition>2314301 - Disclosure - Property, Plant and Equipment (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedPartyTransactions" roleURI="http://www.siga.com/role/RelatedPartyTransactions">
        <link:definition>2112100 - Disclosure - Related Party Transactions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedPartyTransactionsDetails" roleURI="http://www.siga.com/role/RelatedPartyTransactionsDetails">
        <link:definition>2412401 - Disclosure - Related Party Transactions (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringCharges" roleURI="http://www.siga.com/role/RestructuringCharges">
        <link:definition>2118100 - Disclosure - Restructuring Charges</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringChargesDetails" roleURI="http://www.siga.com/role/RestructuringChargesDetails">
        <link:definition>2418402 - Disclosure - Restructuring charges (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringChargesTables" roleURI="http://www.siga.com/role/RestructuringChargesTables">
        <link:definition>2318301 - Disclosure - Restructuring charges (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockCompensationPlans" roleURI="http://www.siga.com/role/StockCompensationPlans">
        <link:definition>2108100 - Disclosure - Stock Compensation Plans</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockCompensationPlansRestrictedStockAwardsAndRestrictedStockUnitsDetails" roleURI="http://www.siga.com/role/StockCompensationPlansRestrictedStockAwardsAndRestrictedStockUnitsDetails">
        <link:definition>2408404 - Disclosure - Stock Compensation Plans (Restricted Stock Awards and Restricted Stock Units) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockCompensationPlansStockAppreciationRightsDetails" roleURI="http://www.siga.com/role/StockCompensationPlansStockAppreciationRightsDetails">
        <link:definition>2408403 - Disclosure - Stock Compensation Plans (Stock Appreciation Rights) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockCompensationPlansStockPlansAndStockOptionsDetails" roleURI="http://www.siga.com/role/StockCompensationPlansStockPlansAndStockOptionsDetails">
        <link:definition>2408402 - Disclosure - Stock Compensation Plans (Stock Plans and Stock Options) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockCompensationPlansTables" roleURI="http://www.siga.com/role/StockCompensationPlansTables">
        <link:definition>2308301 - Disclosure - Stock Compensation Plans (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockCompensationPlansWarrantsDetails" roleURI="http://www.siga.com/role/StockCompensationPlansWarrantsDetails">
        <link:definition>2408405 - Disclosure - Stock Compensation Plans (Warrants) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockholdersEquity" roleURI="http://www.siga.com/role/StockholdersEquity">
        <link:definition>2106100 - Disclosure - Stockholders' Equity</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockholdersEquityDetails" roleURI="http://www.siga.com/role/StockholdersEquityDetails">
        <link:definition>2406401 - Disclosure - Stockholders' Equity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryOfSignificantAccountingPolicies" roleURI="http://www.siga.com/role/SummaryOfSignificantAccountingPolicies">
        <link:definition>2102100 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryOfSignificantAccountingPoliciesAccountsReceivablePropertyPlantAndEquipmentAndNetLossPerShareDetails" roleURI="http://www.siga.com/role/SummaryOfSignificantAccountingPoliciesAccountsReceivablePropertyPlantAndEquipmentAndNetLossPerShareDetails">
        <link:definition>2402403 - Disclosure - Summary of Significant Accounting Policies (Accounts Receivable, Property, Plant and Equipment, and Net Loss Per Share) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryOfSignificantAccountingPoliciesAntiDilutiveSecuritiesExludedFromCalculationOfDilutedLossPerShareDetails" roleURI="http://www.siga.com/role/SummaryOfSignificantAccountingPoliciesAntiDilutiveSecuritiesExludedFromCalculationOfDilutedLossPerShareDetails">
        <link:definition>2402404 - Disclosure - Summary of Significant Accounting Policies (Anti-dilutive Securities Exluded from Calculation of Diluted Loss per Share) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryOfSignificantAccountingPoliciesFairValueOfFinancialInstrumentsDetails" roleURI="http://www.siga.com/role/SummaryOfSignificantAccountingPoliciesFairValueOfFinancialInstrumentsDetails">
        <link:definition>2402405 - Disclosure - Summary of Significant Accounting Policies (Fair Value of Financial Instruments) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryOfSignificantAccountingPoliciesPolicies" roleURI="http://www.siga.com/role/SummaryOfSignificantAccountingPoliciesPolicies">
        <link:definition>2202201 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryOfSignificantAccountingPoliciesRevenueDetails" roleURI="http://www.siga.com/role/SummaryOfSignificantAccountingPoliciesRevenueDetails">
        <link:definition>2402406 - Disclosure - Summary of Significant Accounting Policies Revenue (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryOfSignificantAccountingPoliciesTables" roleURI="http://www.siga.com/role/SummaryOfSignificantAccountingPoliciesTables">
        <link:definition>2302302 - Disclosure - Summary of Significant Accounting Policies (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xsd:appinfo>
  </xsd:annotation>
  <xsd:element id="siga_AccruedExpensesAndOtherCurrentLiabilitiesTableTextBlock" name="AccruedExpensesAndOtherCurrentLiabilitiesTableTextBlock" nillable="true" substitutionGroup="xbrli:item" type="nonnum:textBlockItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_AccruedExpensesAndOtherCurrentLiabilitiesTextBlock" name="AccruedExpensesAndOtherCurrentLiabilitiesTextBlock" nillable="true" substitutionGroup="xbrli:item" type="nonnum:textBlockItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_AmountPlaintiffMayRecover" name="AmountPlaintiffMayRecover" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="instant" />
  <xsd:element abstract="true" id="siga_BalanceSheetParentheticalAbstract" name="BalanceSheetParentheticalAbstract" nillable="true" substitutionGroup="xbrli:item" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_BardaContractMember" name="BardaContractMember" nillable="true" substitutionGroup="xbrli:item" type="nonnum:domainItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ChangeInFairValueOfCommonStockWarrants" name="ChangeInFairValueOfCommonStockWarrants" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element id="siga_CommitmentToInvest" name="CommitmentToInvest" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_CommonStockWarrantsCurrentAndNoncurrentFairValueDisclosure" name="CommonStockWarrantsCurrentAndNoncurrentFairValueDisclosure" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_CommonStockWarrantsCurrentFairValueDisclosure" name="CommonStockWarrantsCurrentFairValueDisclosure" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element id="siga_CommonStockWarrantsNoncurrentFairValueDisclosure" name="CommonStockWarrantsNoncurrentFairValueDisclosure" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element id="siga_ComputerEquipment" name="ComputerEquipment" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element abstract="true" id="siga_ConsolidatedStatementsOfChangesInStockholdersEquityAbstract" name="ConsolidatedStatementsOfChangesInStockholdersEquityAbstract" nillable="true" substitutionGroup="xbrli:item" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ContractReceivableCollected" name="ContractReceivableCollected" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ContractReceivableCollectedDeliveredProducts" name="ContractReceivableCollectedDeliveredProducts" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element id="siga_ContractValueOfDevelopmentAndSupportActivities" name="ContractValueOfDevelopmentAndSupportActivities" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element id="siga_Contractreceivablecollectedresearchanddevelopemntservices" name="Contractreceivablecollectedresearchanddevelopemntservices" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_DebtInstrumentDescriptionofVariableRateBasis1" name="DebtInstrumentDescriptionofVariableRateBasis1" nillable="true" substitutionGroup="xbrli:item" type="num:percentItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_DebtPrincipalPaymentsTableTextBlock" name="DebtPrincipalPaymentsTableTextBlock" nillable="true" substitutionGroup="xbrli:item" type="nonnum:textBlockItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_DeferredTaxAsstsOperatingLossCarryforwardsSubjecttoLimitations" name="DeferredTaxAsstsOperatingLossCarryforwardsSubjecttoLimitations" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_DeferredTaxLiabilitiesCapitalizedContractCosts" name="DeferredTaxLiabilitiesCapitalizedContractCosts" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element abstract="true" id="siga_DocumentAndEntityInformationAbstract" name="DocumentAndEntityInformationAbstract" nillable="true" substitutionGroup="xbrli:item" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_DurationOfEquitablePaymentStreamFollowingFirstCommercialSale" name="DurationOfEquitablePaymentStreamFollowingFirstCommercialSale" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_EffectiveIncomeTaxRateReconciliationLossFromFairValueOfCommonWarrants" name="EffectiveIncomeTaxRateReconciliationLossFromFairValueOfCommonWarrants" nillable="true" substitutionGroup="xbrli:item" type="num:percentItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_ExtendedExpirationPeriodMember" name="ExtendedExpirationPeriodMember" nillable="true" substitutionGroup="xbrli:item" type="nonnum:domainItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_FreeRentPeriod" name="FreeRentPeriod" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_GoodwillPolicyPolicyTextBlock" name="GoodwillPolicyPolicyTextBlock" nillable="true" substitutionGroup="xbrli:item" type="nonnum:textBlockItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_GovernmentContractMaximumNumberOfAdditionalCoursesAvailableToPurchase" name="GovernmentContractMaximumNumberOfAdditionalCoursesAvailableToPurchase" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_GovernmentContractNumberOfCoursesManufacturedUsingFederalFunds" name="GovernmentContractNumberOfCoursesManufacturedUsingFederalFunds" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_GovernmentContractNumberOfCoursesPurchasedUnderModifiedContract" name="GovernmentContractNumberOfCoursesPurchasedUnderModifiedContract" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_GovernmentContractValueOfAward1" name="GovernmentContractValueOfAward1" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_GovernmentContractnumberofcoursesacceptedatnocost" name="GovernmentContractnumberofcoursesacceptedatnocost" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_GovernmentContractnumberofcoursesdelivered" name="GovernmentContractnumberofcoursesdelivered" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_GovernmentContractnumberofcoursesdeliveredatnocost" name="GovernmentContractnumberofcoursesdeliveredatnocost" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_IncreaseInFairValueOfWarrants" name="IncreaseInFairValueOfWarrants" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element id="siga_InitialRentPeriod" name="InitialRentPeriod" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_InventoryAbstract" name="InventoryAbstract" nillable="true" substitutionGroup="xbrli:item" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_InventoryWriteDowns" name="InventoryWriteDowns" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_LabEquipmentMember" name="LabEquipmentMember" nillable="true" substitutionGroup="xbrli:item" type="nonnum:domainItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_LeasePropertyPricePerMonth1" name="LeasePropertyPricePerMonth1" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element abstract="true" id="siga_LicenseAgreementLitigationMember" name="LicenseAgreementLitigationMember" nillable="true" substitutionGroup="xbrli:item" type="nonnum:domainItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_LineOfCreditFacilityBorrowingBasePercentage" name="LineOfCreditFacilityBorrowingBasePercentage" nillable="true" substitutionGroup="xbrli:item" type="num:percentItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_LitigationCostsAwardedPercentage" name="LitigationCostsAwardedPercentage" nillable="true" substitutionGroup="xbrli:item" type="num:percentItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_LoanTerm" name="LoanTerm" nillable="true" substitutionGroup="xbrli:item" type="us-types:durationStringItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_MilestonePayment" name="MilestonePayment" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element id="siga_MonthlyRentalPaymentsAfterFiveYears" name="MonthlyRentalPaymentsAfterFiveYears" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_MonthlyRentalPaymentsFirstFiveYears" name="MonthlyRentalPaymentsFirstFiveYears" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_NetProfitMilestone" name="NetProfitMilestone" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element id="siga_NumberOfActiveContracts" name="NumberOfActiveContracts" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_NumberOfActiveGrants" name="NumberOfActiveGrants" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_NumberOfCoursesToBeDelivered1" name="NumberOfCoursesToBeDelivered1" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_Numberofcoursesdeliveredandaccepted" name="Numberofcoursesdeliveredandaccepted" nillable="true" substitutionGroup="xbrli:item" type="xbrli:integerItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_PatentPreparationFees" name="PatentPreparationFees" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element id="siga_PaymentOfCollateralForCommercialSuretyBond" name="PaymentOfCollateralForCommercialSuretyBond" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element id="siga_PaymentToPostCollateralForSuretyBond" name="PaymentToPostCollateralForSuretyBond" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element id="siga_PercentageOfNetProfitsFromSalesAfterNetProfitMilestoneResultingFromAwardDamages" name="PercentageOfNetProfitsFromSalesAfterNetProfitMilestoneResultingFromAwardDamages" nillable="true" substitutionGroup="xbrli:item" type="num:percentItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_PotentialFutureAggregateResearchAndDevelopmentExpenses" name="PotentialFutureAggregateResearchAndDevelopmentExpenses" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ProceedsFromAdvancePaymentsGovernmentContracts" name="ProceedsFromAdvancePaymentsGovernmentContracts" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element id="siga_ProceedsFromIssuanceOfWarrantsAndOptions" name="ProceedsFromIssuanceOfWarrantsAndOptions" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element id="siga_ProceedsFromIssuanceOfWarrantsAndOptions1" name="ProceedsFromIssuanceOfWarrantsAndOptions1" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ProcurementContractAndResearchAgreementsTextBlock" name="ProcurementContractAndResearchAgreementsTextBlock" nillable="true" substitutionGroup="xbrli:item" type="nonnum:textBlockItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_ProcurementContractAxis" name="ProcurementContractAxis" nillable="true" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_ProcurementContractDomain" name="ProcurementContractDomain" nillable="true" substitutionGroup="xbrli:item" type="nonnum:domainItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_ProcurementContractLineItems" name="ProcurementContractLineItems" nillable="true" substitutionGroup="xbrli:item" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_ProcurementContractTable" name="ProcurementContractTable" nillable="true" substitutionGroup="xbrldt:hypercubeItem" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_QuarterlyFinancialInformationAbstract" name="QuarterlyFinancialInformationAbstract" nillable="true" substitutionGroup="xbrli:item" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ReceivablesPercentage" name="ReceivablesPercentage" nillable="true" substitutionGroup="xbrli:item" type="num:percentItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ReclassOfCommonStockWarrantLiabilityToAdditionalPaidInCapitalUponExercise" name="ReclassOfCommonStockWarrantLiabilityToAdditionalPaidInCapitalUponExercise" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element id="siga_RentExpenseFacilitiesLease" name="RentExpenseFacilitiesLease" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_RentPeriodAfterFirstFiveYears" name="RentPeriodAfterFirstFiveYears" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ResearchAndDevelopmentRevenue" name="ResearchAndDevelopmentRevenue" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element id="siga_ResearchandDevelopmentRevenueLongtermContract" name="ResearchandDevelopmentRevenueLongtermContract" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element id="siga_RestructuringandRelatedCostExpectedExpenseReduction" name="RestructuringandRelatedCostExpectedExpenseReduction" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_RevenueLineItems" name="RevenueLineItems" nillable="true" substitutionGroup="xbrli:item" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element abstract="true" id="siga_RevenueTable" name="RevenueTable" nillable="true" substitutionGroup="xbrldt:hypercubeItem" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ScheduleOfShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsValuationAssumptionsTableTextBlock" name="ScheduleOfShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsValuationAssumptionsTableTextBlock" nillable="true" substitutionGroup="xbrli:item" type="nonnum:textBlockItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ServiceAgreementTerm1" name="ServiceAgreementTerm1" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ShareBasedCompensationArragementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestWeightedAverageExcercisePrice" name="ShareBasedCompensationArragementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestWeightedAverageExcercisePrice" nillable="true" substitutionGroup="xbrli:item" type="num:perShareItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableIntrinsicValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableIntrinsicValue" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableWeightedAverageRemainingLife" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableWeightedAverageRemainingLife" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingIntrinsicValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingIntrinsicValue" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestWeightedAverageRemainingLife" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestWeightedAverageRemainingLife" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThenOptionsExercisableNumber" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThenOptionsExercisableNumber" nillable="true" substitutionGroup="xbrli:item" type="xbrli:sharesItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesThatCouldBeIssued" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesThatCouldBeIssued" nillable="true" substitutionGroup="xbrli:item" type="xbrli:sharesItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice" nillable="true" substitutionGroup="xbrli:item" type="num:perShareItemType" xbrli:periodType="instant" />
  <xsd:element abstract="true" id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePriceRollForward" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePriceRollForward" nillable="true" substitutionGroup="xbrli:item" type="xbrli:stringItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCanceledExpiredWeightedAverageExcercisePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCanceledExpiredWeightedAverageExcercisePrice" nillable="true" substitutionGroup="xbrli:item" type="num:perShareItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExcercisedWeightedAverageExcercisePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExcercisedWeightedAverageExcercisePrice" nillable="true" substitutionGroup="xbrli:item" type="num:perShareItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageExcercisePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageExcercisePrice" nillable="true" substitutionGroup="xbrli:item" type="num:perShareItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsPerformanceBasedNumber" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsPerformanceBasedNumber" nillable="true" substitutionGroup="xbrli:item" type="xbrli:sharesItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsExercisedinPeriod" name="SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsExercisedinPeriod" nillable="true" substitutionGroup="xbrli:item" type="xbrli:sharesItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsExercisablePerformanceBasedNumber" name="SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsExercisablePerformanceBasedNumber" nillable="true" substitutionGroup="xbrli:item" type="xbrli:sharesItemType" xbrli:periodType="instant" />
  <xsd:element id="siga_SharebasedCompensationArrangementbySharebasedPaymentAwardOtherthanOptionsExercisesinPeriodIntrinsicValue" name="SharebasedCompensationArrangementbySharebasedPaymentAwardOtherthanOptionsExercisesinPeriodIntrinsicValue" nillable="true" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_TermExtension" name="TermExtension" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_WarrantsContractrualTerm" name="WarrantsContractrualTerm" nillable="true" substitutionGroup="xbrli:item" type="xbrli:durationItemType" xbrli:periodType="duration" />
  <xsd:element id="siga_WarrantsToPurchasePercentageOfShares" name="WarrantsToPurchasePercentageOfShares" nillable="true" substitutionGroup="xbrli:item" type="num:percentItemType" xbrli:periodType="instant" />
</xsd:schema>