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    <headerData>
        <submissionType>144</submissionType>
        <filerInfo>
            <filer>
                <filerCredentials>
                    <cik>0001198518</cik>
                    <ccc>XXXXXXXX</ccc>
                </filerCredentials>
            </filer>
            <liveTestFlag>LIVE</liveTestFlag>



        </filerInfo>
    </headerData>
    <formData>
        <issuerInfo>
            <issuerCik>0000860413</issuerCik>
            <issuerName>FIRST INTERSTATE BANCSYSTEM INC.</issuerName>
            <secFileNumber>001-34653</secFileNumber>
            <issuerAddress>
                <ns2:street1>401 North 31st Street</ns2:street1>
                <ns2:city>Billings</ns2:city>
                <ns2:stateOrCountry>MT</ns2:stateOrCountry>
                <ns2:zipCode>59101</ns2:zipCode>
            </issuerAddress>
            <issuerContactPhone>(406) 255-5311</issuerContactPhone>
            <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>QUINN T HEYNEMAN</nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
            <relationshipsToIssuer>
                <relationshipToIssuer>Director's son</relationshipToIssuer>
            </relationshipsToIssuer>
        </issuerInfo>
        <securitiesInformation>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <brokerOrMarketmakerDetails>
                <name>Morgan Stanley Smith Barney LLC Executive Financial Services</name>
                <address>
                    <ns2:street1>1 New York Plaza</ns2:street1>
                    <ns2:street2>8th Floor</ns2:street2>
                    <ns2:city>New York</ns2:city>
                    <ns2:stateOrCountry>NY</ns2:stateOrCountry>
                    <ns2:zipCode>10004</ns2:zipCode>
                </address>
            </brokerOrMarketmakerDetails>
            <noOfUnitsSold>250</noOfUnitsSold>
            <aggregateMarketValue>7631.15</aggregateMarketValue>
            <noOfUnitsOutstanding>104529863</noOfUnitsOutstanding>
            <approxSaleDate>02/26/2025</approxSaleDate>
            <securitiesExchangeName>NASDAQ</securitiesExchangeName>
        </securitiesInformation>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>09/08/2016</acquiredDate>
            <natureOfAcquisitionTransaction>Gift from Affiliate</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>John Heyneman</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>Y</isGiftTransaction>
            <donarAcquiredDate>09/08/2016</donarAcquiredDate>
            <amountOfSecuritiesAcquired>250</amountOfSecuritiesAcquired>
            <natureOfPayment>NA</natureOfPayment>
        </securitiesToBeSold>
        <nothingToReportFlagOnSecuritiesSoldInPast3Months>N</nothingToReportFlagOnSecuritiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>BAE-JOHN HEYNEMAN</name>
                <address>
                    <ns2:street1>401 North 31st Street</ns2:street1>
                    <ns2:city>Billings</ns2:city>
                    <ns2:stateOrCountry>MT</ns2:stateOrCountry>
                    <ns2:zipCode>59101</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <saleDate>02/14/2025</saleDate>
            <amountOfSecuritiesSold>1335</amountOfSecuritiesSold>
            <grossProceeds>43461.33</grossProceeds>
        </securitiesSoldInPast3Months>
        <noticeSignature>
            <noticeDate>02/26/2025</noticeDate>
            <planAdoptionDates/>
            <signature>/s/ John M Heyneman Jr</signature>
        </noticeSignature>
    </formData>
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144
0001198518
XXXXXXXX
LIVE
0000860413
FIRST INTERSTATE BANCSYSTEM INC.
001-34653
401 North 31st Street
Billings
MT
59101
(406) 255-5311
QUINN T HEYNEMAN
Director's son
Common
Morgan Stanley Smith Barney LLC Executive Financial Services
1 New York Plaza
8th Floor
New York
NY
10004
250
7631.15
104529863
02/26/2025
NASDAQ
Common
09/08/2016
Gift from Affiliate
John Heyneman
Y
09/08/2016
250
NA
N
BAE-JOHN HEYNEMAN
401 North 31st Street
Billings
MT
59101
Common
02/14/2025
1335
43461.33
02/26/2025
/s/ John M Heyneman Jr