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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2004-05-04</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000720002</issuerCik>
        <issuerName>MAINSOURCE FINANCIAL GROUP</issuerName>
        <issuerTradingSymbol>MSFG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001250696</rptOwnerCik>
            <rptOwnerName>PARKER JOHN C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>621 S. MAIN, P.O. BOX 242</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>VERSAILLES</rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode>47042</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Sr. V.P., Head of Operations</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common</value>
            </securityTitle>
            <transactionDate>
                <value>2004-05-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4335</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>21.10</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14840</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>401-k</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">This total includes shares which were acquired through the Company's Dividend Reinvestment Plan, as well as shares acquired through a 5% stock dividend issued January 9, 2004, and through a 3 for 2 stock split issued by the Company on April 16, 2004.  Summary of Ownership: 401-k, 8,704 shares; IRA, 2,310 shares; and jointly w/spouse, 3,826 shares.  Total Ownership = 14,840 shares.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>John C. Parker</signatureName>
        <signatureDate>2004-05-05</signatureDate>
    </ownerSignature>
</ownershipDocument>

