UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 10-Q (mark one) QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF ☒ 1934 For the quarterly period endedSeptember 30, 2023 OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF ☐ 1934 For the transition period from to Commission File Number: 000-56598 Commission File Number: 1-10499 NORTHWESTERN ENERGY GROUP, INC. NORTHWESTERN CORP (Exact name of registrant as specifi ...