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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-12-31</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001071264</issuerCik>
        <issuerName>JACKSONVILLE BANCORP INC /FL/</issuerName>
        <issuerTradingSymbol>JAXB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001505877</rptOwnerCik>
            <rptOwnerName>CapGen Capital Group IV LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1185 AVENUE OF THE AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 2000</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</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>1185 AVENUE OF THE AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 2000</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</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>0001505878</rptOwnerCik>
            <rptOwnerName>CapGen Capital Group IV LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1185 AVENUE OF THE AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 2000</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</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>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Series B Preferred Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-12-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by CapGen Capital Group IV LP</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Series A Mandatorily Convertible Preferred Stock</value>
                <footnoteId id="F6"/>
            </securityTitle>
            <transactionDate>
                <value>2012-12-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5000000</value>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by CapGen Capital Group IV LP</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Series A Mandatorily Convertible Preferred Stock</value>
                <footnoteId id="F6"/>
            </securityTitle>
            <transactionDate>
                <value>2012-12-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>22530</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22530000</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>22530</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by CapGen Capital Group IV LP</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Series A Mandatorily Convertible Preferred Stock</value>
                <footnoteId id="F6"/>
            </securityTitle>
            <transactionDate>
                <value>2012-12-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>830</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>830000</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>830</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F5"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The shares reported in this row are owned directly by CapGen Capital Group IV LP (&quot;CapGen LP&quot;). 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">As the sole general partner of CapGen LP, CapGen LLC may be deemed to be the indirect beneficial owner of the shares reported in 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="F3">As the managing member of CapGen LLC, Mr. Ludwig may be deemed to be the indirect beneficial owner of the shares reported in this row 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>
        <footnote id="F4">Upon receipt of regulatory approval, the Noncumulative, Nonvoting Perpetual Preferred Stock, Series B (the &quot;Series B Preferred Stock&quot;) of Jacksonville Bancorp, Inc. (the &quot;Issuer&quot;) became exchangeable, on a one-for-one basis, into shares of Series A Mandatorily Convertible, Noncumulative, Nonvoting Perpetual Preferred Stock, Series A (the &quot;Series A Preferred Stock&quot;).  On December 31, 2012, all 5,000 shares of Series B Preferred Stock held by the Reporting Persons were exchanged into 5,000 shares of Series A Preferred Stock.</footnote>
        <footnote id="F5">Mr. Ludwig directly owns the shares reported in this row.</footnote>
        <footnote id="F6">Upon receipt of requisite shareholder approvals, each share of Series A Preferred Stock will convert into 2,000 shares of common stock and/or nonvoting common stock (subject to adjustments), as more fully described in the Series A Preferred Stock Designation.</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 as the managing member of CAPGEN CAPITAL GROUP IV LLC, the general partner of CAPGEN CAPITAL GROUP IV LP</signatureName>
        <signatureDate>2013-01-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Eugene A. Ludwig</signatureName>
        <signatureDate>2013-01-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
