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): July 2, 2026

EOS ENERGY ENTERPRISES, INC.

(Exact name of registrant as specified in its charter)

3920 Park Avenue

Edison, New Jersey 08820

(Address of principal executive offices, including zip code)

Registrant’s telephone number, including area code: (732) 225-8400

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. ☐

Item 8.01 Other Events.

Rights Offering

On July 2, 2026, Eos Energy Enterprises, Inc. (the “Company”) issued a press release announcing the commencement of its previously-announced rights offering pursuant to a shelf registration statement filed on Form S-3 (File No. 333-295819) (the “Registration Statement”) with the Securities and Exchange Commission (“SEC”) on May 13, 2026, and the prospectus supplement relating to the rights offering filed with the SEC on July 2, 2026 (the prospectus supplement together with the accompanying prospectus, the “Prospectus”).

In connection with the rights offering, the Company is filing certain ancillary documents as Exhibits 4.1, 4.2, 4.3, 99.1, 99.2, 99.3, 99.4, 99.5, 99.6, and 99.7 to this Current Report on Form 8-K for the purpose of incorporating such items by reference to the Registration Statement, of which the Prospectus forms a part. The Company is also filing as Exhibit 5.1 the opinion of Davis Polk & Wardwell LLP in connection with the issuance of the subscription rights and the Company’s common stock, par value $0.0001 per share (“Common Stock”), and warrants to purchase shares of Common Stock, issuable upon exercise of such subscription rights.

This Current Report on Form 8-K does not constitute an offer to sell or the solicitation of an offer to buy the securities described herein, nor will there be any sale of such securities in any state or other jurisdiction in which such offer, solicitation, or sale would be unlawful prior to registration or qualification under the securities laws of any such state or jurisdiction. The rights offering will be made only by means of the Prospectus, which can be accessed through the SEC’s website at www.sec.gov. A copy of the Prospectus may also be obtained by contacting the information agent for the rights offering, Sodali & Co., at (203) 658-9400 (banks and brokers), (833) 225-0490 (individuals call toll-free) or EOSE.info@investor.sodali.com.

A copy of the press release related to the matters set forth herein is attached hereto as Exhibit 99.7 and is incorporated herein by reference.

Item 9.01 Financial Statement and Exhibits.

(d) Exhibits

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.