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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 Delaware to Commission File Number : 1-10499 NorthWestern Energy NORTHWESTERN CORP ( Exact name of registrant as specified in its charter ) ( State or other jurisdiction of incorporation or organization ) 3010 W. 69th Street South Dakota ( Address of principal executive offices ) Sioux Falls Registrant's telephone number , including area code : 605-978-2900 46-0172280 ( I.R.S. Employer Identification No. ) 57108 ( Zip Code ) Title of each class Common stock Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) NWE Name of each exchange on which registered Nasdaq Stock Market LLC Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes x No o Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . Yes o No x 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 . Yes x No o 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 during the preceding 12 months ( or for shorter period that the registrant was required to submit such files ) . Yes x No o Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , 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 . Large Accelerated Filer X Accelerated Filer ☐ Non - 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 . Yes o No o 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 . x Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Act ) . Yes No x The aggregate market value of the voting and non - voting common stock held by nonaffiliates of the registrant was $ 2,757,293,172 computed using the last sales price of $ 54.52 per share of the registrant's common stock on June 30 , 2020 , the last business day of the registrant's most recently completed second fiscal quarter . As of February 5 , 2021 , 50,616,211 shares of the registrant's common stock , par value $ 0.01 per share , were outstanding . Documents Incorporated by Reference Certain sections of our Proxy Statement for the 2021 Annual Meeting of Shareholders are incorporated by reference into Part III of this Form 10 - K