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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-03-25</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001320934</issuerCik>
        <issuerName>International Coal Group, Inc.</issuerName>
        <issuerTradingSymbol>ICO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001329534</rptOwnerCik>
            <rptOwnerName>Kitts Samuel Ralph</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O INTERNATIONAL COAL GROUP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>300 CORPORATE CENTRE DRIVE</rptOwnerStreet2>
            <rptOwnerCity>SCOTT DEPOT</rptOwnerCity>
            <rptOwnerState>WV</rptOwnerState>
            <rptOwnerZipCode>25560</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP Business Dev &amp; Asset Mgt</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-03-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>495</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>11.05</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>124171</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Includes (a) a restricted stock (RS) grant of 6,000 shares, the restrictions on 4,500 have lapsed and the restrictions on the remaining 1,500 will lapse on June 30, 2011; (b) a RS grant of 6,000 shares, the restrictions on 4,500 have lapsed of which 495 have been withheld to satisfy tax withholdings, the restrictions on the remaining 1,500 will lapse on March 25, 2012; (c) a RS grant of 29,759 shares, the restrictions on 7,440 have lapsed and the restrictions on the remaining 22,319 will lapse in equal installments of 7,440 on April 30, 2011 and 2012 and 7,439 on April 30, 2013; (d) a RS grant of 12,839 shares, the restrictions will lapse in equal installments of 3,210 on April 30, 2011, 2012 and 2013 and 3,209 on April 30, 2014; (e) a RS grant of 11,068 shares, the restrictions will lapse in equal installments of 2,767 on April 30, 2012, 2013, 2014 and 2015.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Roger L. Nicholson, Attorney in Fact</signatureName>
        <signatureDate>2011-03-29</signatureDate>
    </ownerSignature>
</ownershipDocument>
