Executive Change · Code of Ethics

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of Report (Date of earliest event reported): May 11, 2026 MDU Resources Group, Inc. (Exact name of registrant as specified in its charter) Delaware 1-03480 30-1133956 (State or other jurisdiction of (Commission File Number) (IRS Employer Identification No.) incorporation) 1200 West Century Avenue P.O. Box 5650 Bismarck , North Dakota (Address of principal executive offices) 58506 (Zip Code) Registrant’s telephone number, including area code: ( 701 ) 530-1000 N/A (Former name or former address, if changed since last report.) Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions: ☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)   ☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)   ☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))   ☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c)) Securities registered pursuant to Section 12(b) of the Securities Exchange Act of 1934: Title of each class Trading Symbol(s) Name of each exchange on which registered Common Stock, par value $1.00 per share MDU New York Stock Exchange Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 or Rule 12b-2 of the Securities Exchange Act of 1934. 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. ☐ Item 5.02. Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers. At the annual stockholders’ meeting of MDU Resources Group, Inc. (the “Company”) held on May 11, 2026 (the “Annual Meeting”), the Company’s stockholders approved the MDU Resources Group, Inc. Long-Term Performance-Based Incentive Plan, as amended and restated (the “LTIP”). The material changes to the LTIP were as follows: (1) increased the number of shares of the Company’s common stock available for issuance under the LTIP by 6,564,000 shares, for a total of 15,806,806 shares available for issuance; and (2) expanded eligibility for a director emeritus to participate in the LTIP. The LTIP became effective immediately upon stockholder approval at the Annual Meeting. Each of Nicole A. Kivisto, President and Chief Executive Officer; Jason L. Vollmer, Chief Financial Officer; Garret Senger, Chief Utilities Officer; Anthony D. Foti, Chief Legal Officer and Corporate Secretary; and Rob L. Johnson, President, WBI Energy, Inc., collectively referred to as the Company’s named executive officers (the “NEOs”), as well as other officers of the Company, participate in the LTIP. A summary of the material terms of the LTIP is set forth in the Company’s definitive proxy statement for the Annual Meeting filed with the U.S. Securities and Exchange Commission on April 1, 2026 (the “Proxy Statement”). The summaries of the LTIP set forth above, and in the Proxy Statement, are qualified in their entirety by reference to the full text of the LTIP, a copy of which is filed as Exhibit 10.1 to this Current Report on Form 8-K, and incorporated herein by reference. Item 5.05. Amendments to the Registrant’s Code of Ethics, or Waiver of a Provision of the Code of Ethics. On May 13, 2026, the Company’s Board of Directors (the “Board”) adopted an updated and revised code of business conduct, referred to as the “Leading With Integrity Policy” (the “Policy”), as part of the Company’s periodic review process. This Policy applies to all non-employee directors, director emeriti, officers, and employees of the Company and its subsidiaries. The updates and revisions to the Policy, among other things: (i) clarify and revise provisions relating to artificial intelligence, confidential information, conflicts of interest, physical security, and regulatory compliance, and (ii) make other non-substantive administrative and stylistic changes. The summary description of these updates and revisions to the Policy set forth above is qualified in its entirety by reference to the full text of the Policy, as amended, which can be found on the Company’s corporate website at investor.mdu.com/governance/governance-documents/. The contents on the Company’s corporate website are not incorporated by reference in this report. Item 5.07. Submission of Matters to a Vote of Security Holders. On May 11, 2026, the Company held the Annual Meeting in a virtual-only format and stockholders voted on the four proposals set forth below. The final voting results are listed below. Proposal 1: Election of Directors . Stockholders elected the eight nominees to the Board for one-year terms expiring at the annual stockholders’ meeting to be held in 2027, based upon the votes set forth in the table below : Name Votes For Votes Against Abstentions Broker Non-Votes Darrel T. Anderson 156,497,374  1,772,995  207,111  25,924,878  Vernon A. Dosch 157,748,991  508,798  219,691  25,924,878  Marian M. Durkin 156,077,464  1,507,033  892,983  25,924,878  Douglas W. Jaeger 157,108,513  1,149,629  219,338  25,924,878  Charles M. Kelley 157,868,483  382,487  226,510  25,924,878  Nicole A. Kivisto 157,782,494  498,930  196,056  25,924,878  Tammy J. Miller 157,313,621  952,917  210,942  25,924,878  Priti R. Patel 157,647,380  608,760  221,340  25,924,878  2 Proposal 2: Advisory Vote to Approve Executive Compensation . Stockholders approved, on an advisory basis, the compensation of the NEOs, based upon the votes set forth in the table below: Votes For Votes Against Abstentions Broker Non-Votes 155,483,583 2,448,238  545,659  25,924,878  Proposal 3: Approval of LTIP . Stockholders approved the LTIP, based upon the votes set forth in the table below: Votes For Votes Against Abstentions Broker Non-Votes 154,587,051 2,817,302  1,073,127  25,924,878  Proposal 4: Ratification of Appointment of Independent Registered Public Accounting Firm . Stockholders ratified the appointment of Deloitte & Touche LLP as the Company’s independent registered public accounting firm for fiscal year 2026, based upon the votes set forth in the table below: Votes For Votes Against Abstentions 173,965,507 10,089,093  347,758  Item 9.01. Financial Statements and Exhibits. (d) Exhibits. Exhibit No. Description 10.1 MDU Resources Group, Inc. Long-Term Performance-Based Incentive Plan, as amended and restated. 104 Cover Page Interactive Data File (embedded within the Inline XBRL document). 3 SIGNATURE Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized. Date: May 13, 2026 MDU Resources Group, Inc. By: /s/ Anthony D. Foti Name: Anthony D. Foti Title: Chief Legal Officer and Corporate Secretary 4