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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-12-08</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001094831</issuerCik>
        <issuerName>BGC Partners, Inc.</issuerName>
        <issuerTradingSymbol>BGCP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001246238</rptOwnerCik>
            <rptOwnerName>SLOANE BARRY R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5 SPINNAKER DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2>PO BOX 247</rptOwnerStreet2>
            <rptOwnerCity>BARRINGTON</rptOwnerCity>
            <rptOwnerState>RI</rptOwnerState>
            <rptOwnerZipCode>02806</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock, par value per $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2008-12-08</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>9916</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9916</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">These 9,916 shares of Class A Common Stock are represented by restricted stock units granted to Mr. Sloane, on December 8, 2008, under the BGC Partners, Inc. Amended and Restated Long Term Incentive Plan.  Each restricted stock unit represents a contingent right to receive one share of BGC Partners, Inc. Class A Common Stock.</footnote>
        <footnote id="F2">The 9,916 restricted stock units will vest on December 8, 2009, provided that Mr. Sloane continues to serve as a member of the BGC Partners, Inc. Board of Directors on such date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Barry R. Sloane</signatureName>
        <signatureDate>2008-12-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
