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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-12-18</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001651052</issuerCik>
        <issuerName>Focus Financial Partners Inc.</issuerName>
        <issuerTradingSymbol>FOCS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001746236</rptOwnerCik>
            <rptOwnerName>Adolf Ruediger</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>825 THIRD AVENUE, 27TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>CEO and Chairman</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Incentive Units in Focus Financial Partners, LLC</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>28.50</value>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2018-12-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>331642</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>331642</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Each incentive unit in Focus Financial Partners, LLC (&quot;Focus LLC&quot;) entitles the holder to receive distributions from Focus LLC if the aggregate distributions made by Focus LLC in respect of each common unit issued and outstanding on or prior to date of the grant of the incentive unit exceeds a specified amount, referred to as the hurdle amount. The hurdle amount is set at the time of grant and typically represents the estimated fair value of a common unit on the date of grant. The figure reflected in column 2 is the hurdle amount assigned to each incentive award. Incentive units do not expire.</footnote>
        <footnote id="F2">Unvested incentive units will, at vesting, become exchangeable, subject to certain restrictions in the Fourth Amended and Restated Operating Agreement of Focus LLC, for (i) a number of shares of the Issuer's Class A common stock that takes into account the then-current value of the Issuer's Class A common stock and such incentive units' aggregate hurdle amount or, (ii) at the election of the Issuer, cash. The acquisition of the derivative securities was exempt from Section 16 of the Securities Exchange Act of 1934, as amended.</footnote>
        <footnote id="F3">These incentive units will vest in four equal installments on each anniversary of December 18, 2018.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ J. Russell McGranahan as Attorney-in-Fact</signatureName>
        <signatureDate>2018-12-20</signatureDate>
    </ownerSignature>
</ownershipDocument>
