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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-11-27</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001644378</issuerCik>
        <issuerName>RMR Group Inc.</issuerName>
        <issuerTradingSymbol>NASDAQ:RMR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001537667</rptOwnerCik>
            <rptOwnerName>Select Income REIT</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TWO NEWTON PLACE</rptOwnerStreet1>
            <rptOwnerStreet2>255 WASHINGTON STREET, SUITE 300</rptOwnerStreet2>
            <rptOwnerCity>NEWTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02458</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock, par value $0.001</value>
            </securityTitle>
            <transactionDate>
                <value>2015-11-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1580093</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1586798</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On November 13, 2015, the Reporting Person approved the distribution of shares of Class A Common Stock of The RMR Group Inc. (&quot;INC Shares&quot;) pro rata to holders of the Reporting Person's common shares of beneficial interest of record as of November 27, 2015 (the &quot;Distribution&quot;).  The Distribution is payable on December 14, 2015, subject to the registration statement on Form S-1 for the Distribution remaining effective through such date and the INC Shares remaining listed on the NASDAQ Stock Market LLC through such date.  The number of INC Shares being distributed by the Reporting Person is subject to adjustment upon confirmation by the Depositary Trust Company (&quot;DTC&quot;) of required payments of cash in lieu of fractional shares to DTC participants.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ John C. Popeo, Treasurer and Chief Financial Officer, on behalf of Select Income REIT</signatureName>
        <signatureDate>2015-12-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
