<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-09-18</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001369786</issuerCik>
        <issuerName>Sagent Pharmaceuticals, Inc.</issuerName>
        <issuerTradingSymbol>SGNT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001230193</rptOwnerCik>
            <rptOwnerName>MORGAN STANLEY INVESTMENT MANAGEMENT INC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>522 FIFTH AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>4TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001230207</rptOwnerCik>
            <rptOwnerName>MORGAN STANLEY AIP GP LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>100 FRONT STREET</rptOwnerStreet1>
            <rptOwnerStreet2>STE 400</rptOwnerStreet2>
            <rptOwnerCity>WEST CONSHOHOCKEN</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19428</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2013-09-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>49043</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.8366</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4239799</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2013-09-19</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>39842</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.148</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4199957</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares of Common Stock owned by certain funds (each, a &quot;Fund&quot; and collectively, the &quot;Funds&quot;) and certain separately managed accounts managed by the undersigned.</footnote>
        <footnote id="F2">Updated to correct previously reported total.</footnote>
        <footnote id="F3">The shares reported are held of record by Stormlaunch &amp; Co., Sailorshell &amp; Co., Stormbay &amp; Co., Stormstar &amp; Co., Weyerhaeuser Company Master Retirement Trust, Sailorpier &amp; Co., Factory Mututal Insurance Company and Nuclear Electrical Insurance Limited.</footnote>
        <footnote id="F4">The reported securities are held directly by the Funds and certain separately managed accounts managed by each of the reporting persons. Morgan Stanley Investment Management Inc. (&quot;MSIM&quot;) is the investment manager of one or more of the separately managed accounts and Morgan Stanley AIP GP LP (&quot;AIP&quot;) is the investment manager of one or more of the Funds.  This form is filed without prejudice to the reporting persons' position that none of the Funds nor any subsidiaries of the reporting persons, nor the reporting persons individually or in the aggregate, are required to file beneficial ownership reports under Section 16(a) of the Securities Exchange Act of 1934, and should be not construed or interpreted as a concession or admission that such reports are required.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Mary Ann Picciotto</signatureName>
        <signatureDate>2013-09-20</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Mary Ann Picciotto</signatureName>
        <signatureDate>2013-09-20</signatureDate>
    </ownerSignature>
</ownershipDocument>
