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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-12-21</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001592386</issuerCik>
        <issuerName>Virtu Financial, Inc.</issuerName>
        <issuerTradingSymbol>VIRT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001602512</rptOwnerCik>
            <rptOwnerName>TJMT Holdings LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O VIRTU FINANCIAL, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>900 3RD AVENUE</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle>See Viola, Vincent J.</officerTitle>
            <otherText>See Remarks</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001573310</rptOwnerCik>
            <rptOwnerName>Viola Michael T</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O VIRTU FINANCIAL, INC., 900 THIRD AVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001602918</rptOwnerCik>
            <rptOwnerName>Viola Teresa</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O VIRTU FINANCIAL, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>900 3RD AVENUE</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>See Remarks</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001573350</rptOwnerCik>
            <rptOwnerName>Viola Vincent J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>900 THIRD AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022-1010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Executive Chairman</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A common stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-12-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>50000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>15.7814</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>326624</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class A common stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>200000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class A common stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3407</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F5"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">This transaction was effected pursuant to a Rule 10b5-1 Plan adopted by TJMT Holdings LLC on November 10, 2016 which provides for the purchase of up to 1,000,000 shares of Class A Common Stock.  Accordingly, the reporting person had no discretion with regard to the timing of the transaction.</footnote>
        <footnote id="F2">The price reported in Column 4 is a weighted average price. The shares were purchased in multiple transactions at prices ranging from $15.65 to $15.85, inclusive. The reporting person undertakes to provide to the staff of the Securities and Exchange Commission, to any security holder of Virtu Financial, Inc., or to Virtu Financial, Inc., upon request, full information regarding the number of shares purchased at each separate price within the range set forth above.</footnote>
        <footnote id="F3">Mr. Michael Viola and Mrs. Teresa Viola each have an indirect interest in the securities acquired by TJMT Holdings LLC, a Viola family vehicle over which the reporting persons shares dispositive control and voting control. Mr. Vincent Viola has an indirect interest in the securities acquired by TJMT Holdings LLC by virtue of his spouse's dispositive and voting control over TJMT Holdings LLC.</footnote>
        <footnote id="F4">Directly owned by Mr. Vincent Viola.</footnote>
        <footnote id="F5">Directly owned by Mr. Michael Viola.</footnote>
    </footnotes>

    <remarks>By virtue of relationship with Mr. Vincent Viola, a director of the Issuer, Mrs. Teresa Viola and TJMT Holdings LLC may each be deemed to be a director by deputization.</remarks>

    <ownerSignature>
        <signatureName>/s/ Justin Waldie, as Attorney-in-Fact for TJMT Holdings LLC</signatureName>
        <signatureDate>2016-12-21</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Justin Waldie, as Attorney-in-Fact for Michael Viola</signatureName>
        <signatureDate>2016-12-21</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Justin Waldie, as Attorney-in-Fact for Teresa Viola</signatureName>
        <signatureDate>2016-12-21</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Justin Waldie, as Attorney-In-Fact for Vincent Viola</signatureName>
        <signatureDate>2016-12-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
