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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2004-07-13</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001059324</issuerCik>
        <issuerName>PETROHAWK ENERGY CORP</issuerName>
        <issuerTradingSymbol>HAWK</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001290913</rptOwnerCik>
            <rptOwnerName>EnCap Equity Fund IV GP, LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1100 LOUISIANA</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 3150</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77002</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</value>
            </securityTitle>
            <transactionDate>
                <value>2004-07-13</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>15000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7592567</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by EnCap Energy Capital Fund IV L.P. and PHAWK LLC</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2004-10-28</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1190</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>8.40</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7592567</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by EnCap Energy Capital Fund IV L.P. and PHAWK LLC</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents 7,500 shares of Common Stock granted to David B. Miller as compensation for David B. Miller serving on the Issuer's board of directors and assigned by David B. Miller to EnCap Energy Capital Fund IV, L.P. and 7,500 shares of Common Stock granted to D. Martin Phillips as compensation for D. Martin Phillips serving on the Issuer's board of directors and assigned by D. Martin Phillips to EnCap Energy Capital Fund IV, L.P.</footnote>
        <footnote id="F2">Represents 595 shares of Common Stock granted to EnCap Energy Capital Fund IV, L.P. as compensation for David B. Miller serving on the Issuer's board of directors and 595 shares of Common Stock granted to EnCap Energy Capital Fund IV, L.P. as compensation for D. Martin Phillips serving on the Issuer's board of directors.</footnote>
        <footnote id="F3">Represents 16,810 shares owned by EnCap Energy Capital Fund IV, L.P. and 7,575,757 shares owned by PHAWK, LLC (formerly Petrohawk Energy, LLC).  The reporting person disclaims any beneficial ownership of the securities owned by PHAWK, LLC or EnCap Energy Capital Fund IV, L.P. in excess of its pecuniary interest in such securities.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>EnCap Equity Fund IV GP, L.P., by EnCap Investments L.P., its general partner, by EnCap Investments GP, L.L.C., its general partner, by /s/ David B. Miller, Senior Managing Director</signatureName>
        <signatureDate>2004-10-29</signatureDate>
    </ownerSignature>
</ownershipDocument>

