Exhibit 5.1

May 17, 2023

Eos Energy Enterprises, Inc.

3920 Park Avenue

Edison, NJ 08820

Ladies and Gentlemen:

We have acted as counsel to Eos Energy Enterprises, Inc., a Delaware corporation (the “Company”), in connection with the filing with the Securities and Exchange Commission (the “Commission”) pursuant to Rule 424(b) under the Securities Act of 1933, as amended (the “Act”), of the Company’s prospectus supplement, dated May 15, 2023 (the “Prospectus Supplement”), forming part of the registration statement on Form S-3, initially filed by the Company with the Commission on March 4, 2022, and declared effective on April 25, 2022 (the “Registration Statement”). The Prospectus Supplement relates to the proposed sale of 3,601,980 shares (the “Shares”) of the Company’s common stock, par value $0.0001 per share (the “Common Stock”), pursuant to that certain Securities Purchase Agreement, dated May 15, 2023, by and between the Company and each purchaser identified on the signature pages thereto (the “Securities Purchase Agreement”). 

For purposes of the opinion we express below, we have examined originals, or copies certified or otherwise identified, of (i) the Third Amended and Restated Certificate of Incorporation (the “Certificate of Incorporation”) and the Amended and Restated Bylaws (the “Bylaws”) of the Company, each as amended and/or restated as of the date hereof; (ii) certain resolutions of the Board of Directors of the Company approving the filing of the Registration Statement and Base Prospectus (as defined below) and certain other matters set forth therein; (iii) certain resolutions of the Board of Directors of the Company related to the filing of the Prospectus Supplement, the authorization and issuance of the Shares and related matters; (iv) the Registration Statement and all exhibits thereto; (v) the Prospectus Supplement and the prospectus included in the Registration Statement dated April 25, 2022 (the “Base Prospectus” and together with the Prospectus Supplement, the “Prospectus”); (vi) the Securities Purchase Agreement; (vii) a certificate executed by an officer of the Company, dated as of the date hereof; (viii) the order of the Court of Chancery for the State of Delaware issued on February 27, 2023 regarding the Section 205 petition filed with the Court of Chancery for the State of Delaware on February 10, 2023 and (ix) such other corporate records of the Company, as we have deemed necessary or appropriate for the purposes of the opinions hereafter expressed.

As to questions of fact material to the opinions expressed below, we have, without independent verification of their accuracy, relied to the extent we deem reasonably appropriate upon the representations and warranties of the Company contained in such documents, records, certificates, instruments or representations furnished or made available to us by the Company.

In making the foregoing examination, we have assumed (i) the genuineness of all signatures, (ii) the authenticity of all documents submitted to us as originals, (iii) the conformity to original documents of all documents submitted to us as certified or photostatic copies, (iv) that all agreements or instruments we have examined are the valid, binding and enforceable obligations of the parties thereto, and (v) that all factual information on which we have relied was accurate and complete.

We have also assumed that, at the time of the issuance of the Shares: (i) the resolutions of the Board of Directors of the Company referred to above will not have been modified or rescinded, (ii) the Company will receive consideration for the issuance of the Shares as set forth in the Securities Purchase Agreement which shall be at least equal to the par value of the Common Stock and (iii) all requirements of the General Corporation Law of the State of Delaware (the “DGCL”), the Certificate of Incorporation and the Bylaws will be complied with when the Shares are issued.

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Based on the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that the Shares, upon payment and delivery in accordance with the Securities Purchase Agreement, will be validly issued, fully paid and non-assessable.

We hereby consent to the filing of this opinion as Exhibit 5.1 to the Company’s Current Report on Form 8-K to be filed with the Commission. We further consent to the reference to our firm under the caption “Legal Matters” in the Prospectus on the date hereof constituting a part of the Registration Statement. In giving this consent, we are not admitting that we are within the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder. This opinion is given as of the date hereof and we assume no obligation to update or supplement such opinion after the date hereof to reflect any facts or circumstances that may thereafter come to our attention or any changes that may thereafter occur. The opinions expressed herein are limited exclusively to the DGCL and we have not considered, and express no opinion on, any other laws or the laws of any other jurisdiction.

Very truly yours,

/s/ Haynes and Boone, LLP

HAYNES AND BOONE, LLP