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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-06-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001373561</issuerCik>
        <issuerName>NewStar Financial, Inc.</issuerName>
        <issuerTradingSymbol>NEWS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001408853</rptOwnerCik>
            <rptOwnerName>Corsair Capital LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>717 FIFTH AVENUE, 24TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2014-06-03</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>X</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>200174</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.90</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9704972</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2014-06-03</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>166557</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>13.10</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9538415</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Warrant (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>10.90</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2014-06-03</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>X</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>200174</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>200174</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Reflects the exercise of a warrant held directly by Corsair II Capital Partners, L.P. (f/k/a J.P. Morgan Corsair II Capital Partners, L.P.) (&quot;Corsair II&quot;).</footnote>
        <footnote id="F2">Consists of (a) 5,538,415 shares held directly by Corsair II, (b) 3,812,268 shares held directly by  Corsair III Financial Services Capital Partners, L.P. (&quot;Corsair III&quot;), and (c) 187,732 shares held directly by Corsair III Financial Services Offshore 892 Partners, L.P. (&quot;Corsair III Offshore&quot;).</footnote>
        <footnote id="F3">Reflects the withholding of shares to pay the exercise price in connection with the exercise of the warrant reported herein.</footnote>
        <footnote id="F4">Corsair III Management, L.P. is the General Partner of Corsair III and Corsair III Offshore. The Reporting Person is the General Partner of Corsair III Management, L.P.  The General Partner of Corsair II is Corsair II, L.P., the General Partner of Corsair II, L.P. is Corsair II, L.L.C., and the Managing Member of Corsair II, L.L.C. is Corsair PTJB, LLC. The Reporting Person is the Manager of Corsair PTJB, LLC.</footnote>
        <footnote id="F5">The Reporting Person is filing solely in its capacity as General Partner of Corsair III Management, L.P. and as Manager of Corsair PTJB, LLC. By reason of the provisions of Rule 13d-3(a) of the Securities Exchange Act of 1934, the Reporting Person and Corsair III Management L.P. may be deemed to beneficially own the securities held by Corsair III and Corsair III Offshore, although each of the Reporting Person and Corsair III Management L.P., respectively, disclaims beneficial ownership of such securities except to the extent of their respective pecuniary interests therein. (Text continues in Note 6).</footnote>
        <footnote id="F6">(Text continued from Note 5) By reason of the provisions of Rule 13d-3(a) of the Securities Exchange Act of 1934, the Reporting Person, Corsair II, L.P., Corsair II, L.L.C. and Corsair PTJB, LLC may be deemed to beneficially own the securities held by Corsair II, although each of the Reporting Person, Corsair II, L.P., Corsair II, L.L.C. and Corsair PTJB, LLC, respectively, disclaims beneficial ownership of such securities except to the extent of their respective pecuniary interests therein. The Reporting Person holds the securities indirectly through Corsair II, Corsair III and Corsair III Offshore.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Amy Knapp, Chief Financial Officer and Chief Operating Officer</signatureName>
        <signatureDate>2015-04-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
