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( Mark One ) UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington D.C. 20549 FORM 10 - K Annual Report Pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 For the fiscal year ended December 31 , 2020 or Transition Report Pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 For the transition period from to Montana ( State or other jurisdiction of incorporation or organization ) FIRST INTERSTATE BANCSYSTEM , INC . ( Exact name of registrant as specified in its charter ) 401 North 31st Street Billings , MT ( Address of principal executive offices ) Title of each class Class A common stock , no par value Commission File Number : 001-34653 ( 406 ) 255-5390 ( Registrant's telephone number , including area code ) Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) FIBK Class A common stock Class B common stock Securities registered pursuant to Section 12 ( g ) of the Act : Class B common stock ( Title of class ) 81-0331430 ( IRS Employer Identification No. ) Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . p Yes o No Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . o Yes þ No Indicate by check mark whether the registrant : ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . þ Yes o No 59116 ( zip code ) Name of exchange on which registered NASDAQ Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §223.405 of this chapter ) during the preceding 12 months ( or for such shorter period that registrant was required to submit such files ) . þ Yes o No ☐ ☐ Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of " large accelerated filer , ” “ accelerated filer , ” “ smaller reporting company " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . ( Check one ) : Large accelerated filer Accelerated filer Smaller reporting company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . 0 Non - accelerated filer Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . Indicate by check mark if the registrant is a shell company ( as defined in Rule 12b - 2 of the Act . ) Yes þ No The aggregate market value of voting and non - voting common equity held by non - affiliates , computed by reference to the price at which the common equity was last sold on the NASDAQ , as of the last business day of the registrant’s most recently completed second fiscal quarter was $ 1,379,587,569 . Indicate the number of shares outstanding of each of the registrant's classes of common stock as of January 31 , 2021 : 40,942,784 21,130,474 Documents Incorporated by Reference The registrant intends to file a definitive Proxy Statement for the Annual Meeting of Shareholders scheduled to be held May 26 , 2021. The information required by Part III of this Form 10 - K is incorporated by reference to such Proxy Statement .