8-K

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): January 23, 2026

UNITIL CORPORATION

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (603) 772-0775

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:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

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. ☐

Amendment to Purchase Agreement

On May 6, 2025, Unitil Corporation, a New Hampshire corporation (the “Company”), entered into a Purchase and Sale Agreement (the “Purchase Agreement”) by and between the Company and Aquarion Water Authority, a public corporation and political subdivision of the state of Connecticut (“Seller”), and, solely with respect to Section 9.25 and Section 9.26 thereof, South Central Connecticut Regional Water Authority (“RWA”). Pursuant to the Purchase Agreement, the Company agreed to acquire all of the issued and outstanding shares of common stock of Aquarion Water Company of Massachusetts, Inc., Aquarion Water Company of New Hampshire, Inc., and Abenaki Water Co., Inc. from Seller (the “Acquisition”).

The Company previously disclosed a brief summary of the material terms and conditions of the Purchase Agreement in a Form 8-K dated May 6, 2025, which the Company filed with the Securities and Exchange Commission on May 12, 2025.

On January 23, 2026, the Company entered into an Amendment No. 1 to Purchase and Sale Agreement (the “Amendment”) by and between the Company, Seller and RWA. The Amendment, among other things, amends the definition of “Termination Date” in Section 8.1(d) of the Purchase Agreement by replacing the reference to “January 23, 2026” with “February 23, 2026”. Except as specifically modified and amended by the Amendment, the Purchase Agreement remains unchanged.

Neither the Company nor any of its controlled affiliates has any material relationship with Seller or RWA, other than with respect to the Purchase Agreement and the Amendment.

The foregoing summary of the Amendment does not purport to be complete and is subject to, and qualified in its entirety by, the full text of the Amendment attached hereto as Exhibit 2.1.

The Amendment has been included to provide investors with information regarding its terms. The Amendment is not intended to provide any factual information about the Company. The representations, warranties and covenants contained in the Amendment were made only for purposes of the Amendment and as of specific dates and were solely for the benefit of the parties to the Amendment. Moreover, the representations and warranties contained in the Amendment generally were made for the purpose of allocating contractual risk among the parties to the Amendment instead of establishing matters as facts, and may be subject to standards of materiality applicable to the parties to the Amendment that differ from those applicable to investors. Investors are not third-party beneficiaries under the Amendment and should not rely on the representations, warranties and covenants contained therein or any descriptions thereof as characterizations of the actual state of facts or condition of the Company or any of its subsidiaries or affiliates. Additionally, information concerning the subject matter of the representations and warranties contained in the Amendment may change after the date of the Amendment, which subsequent information may or may not be fully reflected in the Company’s public disclosures.

(d) Exhibits

SIGNATURES

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: January 29, 2026

EX-2.1

Exhibit 2.1

EXECUTION VERSION

AMENDMENTNO. 1 TO PURCHASE AND SALE AGREEMENT

This Amendment No. 1 to Purchase and Sale Agreement (this “Amendment”), dated as of January 23, 2026 (the “Effective Date”), is by and among Unitil Corporation, a New Hampshire corporation (“Buyer”), Aquarion Water Authority, a public corporation and political subdivision of the state of Connecticut (“Seller”), and South Central Connecticut Regional Water Authority (“RWA”). Buyer, Seller and RWA will be referred to herein from time to time collectively as the “Parties”. Each capitalized term used and not defined herein has the meaning set forth in the Purchase and Sale Agreement.

WHEREAS, the Parties are parties to that certain Purchase and Sale Agreement, dated as of May 6, 2025 (the “Agreement”);

WHEREAS, subject to the terms and conditions set forth in this Amendment, and pursuant to Section 9.19 of the Agreement, the Parties desire to amend certain terms of the Agreement by entering into this Amendment.

NOW,THEREFORE, for good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, and intending to be legally bound, the Parties agree as follows:

[Signature Page Follows]

IN WITNESS WHEREOF, each of the undersigned has caused this Amendment to be duly executed on its behalf as of the day and year first above written.

[Signature Page to Amendment No. 1 to Purchase and Sale Agreement]