Exhibit 5.1

August 15, 2025

Plug Power Inc.

125 Vista Boulevard

Slingerlands, NY 12159

Re:         Securities Registered under Registration Statement on Form S-3

We have acted as counsel to you in connection with your filing of a Registration Statement on Form S-3 (File No. 333-287577) (as amended or supplemented, the “Registration Statement”) filed on May 27, 2025 with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration by Plug Power Inc., a Delaware corporation (the “Company”), of any combination of securities of the types specified therein. The Registration Statement became effective upon filing with the Commission on May 27, 2025. Reference is made to our opinion letter dated May 27, 2025 and included as Exhibit 5.1 to the Registration Statement. We are delivering this supplemental opinion letter in connection with the prospectus supplement (the “Prospectus Supplement”) filed on August 15, 2025 by the Company with the Commission pursuant to Rule 424 under the Securities Act. The Prospectus Supplement relates to the offering by the Company of up to $1,000,000,000 in aggregate offering price of shares (the “Shares”) of the Company’s common stock, par value $0.01 per share (“Common Stock”), covered by the Registration Statement. The Shares are being offered and sold by the sales agent named in, and pursuant to, the At Market Issuance Sales Agreement, dated as of January 17, 2024, as amended on February 23, 2024, November 7, 2024, and August 15, 2025, between the Company and such sales agent (the “Sales Agreement”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinion set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

For purposes of the opinion set forth below, we have assumed that the Shares are issued for a price per share equal to or greater than the minimum price authorized by the Company’s board of directors prior to the date hereof (the “Minimum Price”), that no event occurs that causes the number of authorized shares of Common Stock available for issuance by the Company to be less than the number of then unissued Shares that may be issued for the Minimum Price and that the total number of shares of Common Stock issued and outstanding, after giving effect to the issuance of Shares under the Sales Agreement, will not exceed the total number of shares of Common Stock that the Company has authority to issue under its Amended and Restated Certificate of Incorporation, as amended.

Plug Power Inc.

August 15, 2025

Page 2

For purposes of the opinion set forth below, we refer to the following as “Future Approval and Issuance”: (a) the approval by the Company’s board of directors (or a duly authorized committee of the board of directors) of the issuance of the Shares (the “Approval”) and (b) the issuance of the Shares in accordance with the Approval and the receipt by the Company of the consideration (which shall not be less than the par value of such Shares) to be paid in accordance with the Approval.

The opinion set forth below is limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, upon Future Approval and Issuance, will be validly issued, fully paid and nonassessable

This opinion letter and the opinion it contains shall be interpreted in accordance with the Core Opinion Principles as published in 74 Business Lawyer 815 (Summer 2019).

This opinion letter is being furnished to you for submission to the Commission as an exhibit to the Company’s Current Report on Form 8-K relating to the future issuance of the Shares (the “Current Report”), which is incorporated by reference in the Registration Statement. We hereby consent to the filing of this opinion letter as an exhibit to the Current Report and its incorporation by reference and the reference to our firm in that report. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.