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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-11-16</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001005507</issuerCik>
        <issuerName>JACKSONVILLE BANCORP INC</issuerName>
        <issuerTradingSymbol>JAXB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001505877</rptOwnerCik>
            <rptOwnerName>CapGen Capital Group IV LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>280 PARK AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>40TH FLOOR WEST SUITE 401</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10017</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>0001453863</rptOwnerCik>
            <rptOwnerName>Ludwig Eugene</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>280 PARK AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>40TH FLOOR WEST SUITE 401</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10017</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>0001505878</rptOwnerCik>
            <rptOwnerName>CapGen Capital Group IV LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>280 PARK AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>40TH FLOOR WEST SUITE 401</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10017</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</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>2010-11-16</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2684144</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>9.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by GapGen Capital Group IV LP</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">CapGen Capital Group IV LP (&quot;CapGen LP&quot;) acquired the shares of common stock, par value $0.01 per share (the &quot;Common Stock&quot;), of Jacksonville Bancorp, Inc. (the &quot;Issuer&quot;) on November 16, 2010. CapGen Capital Group IV LLC (&quot;CapGen LLC&quot;) is the sole general partner of CapGen LP. Mr. Eugene A. Ludwig is the managing member of CapGen LLC.</footnote>
        <footnote id="F2">CapGen LP directly owns the shares of Common Stock on this row.</footnote>
        <footnote id="F3">As the sole general partner of CapGen LP, CapGen LLC may be deemed to be the indirect beneficial owner of the shares of Common Stock on this row under Rule 16a-1(a)(2) promulgated under the Securities Exchange Act of 1934, as amended (the &quot;Exchange Act&quot;). Pursuant to Rule 16a-1(a)(4) promulgated under the Exchange Act, CapGen LLC disclaims that it is the beneficial owner of such shares, except to the extent of its pecuniary interest.</footnote>
        <footnote id="F4">As the managing member of CapGen LLC, Mr. Ludwig may be deemed to be the indirect beneficial owner of the shares of Common Stock under Rule 16a-1(a)(2) promulgated under the Exchange Act. Pursuant to Rule 16a-1(a)(4) promulgated under the Exchange Act, Mr. Ludwig disclaims that he is the beneficial owner of such shares, except to the extent of his pecuniary interest.</footnote>
    </footnotes>

    <remarks>CapGen LP, CapGen LLC and Mr. Ludwig disclaim their status as directors by deputization by virtue of Messrs. John Rose's and John Sullivan's positions as members of the board of directors of the Issuer.</remarks>

    <ownerSignature>
        <signatureName>/s/ Eugene A. Ludwig, the managing member of CapGen Capital Group IV LLC, the general partner of CapGen Capital Group IV LP</signatureName>
        <signatureDate>2010-11-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
