Exhibit 5.1

March 6, 2014

Plug Power Inc.

968 Albany Shaker Road

Latham, New York 12110

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

Ladies and Gentlemen:

We have acted as counsel to you in connection with your filing of (i) a Registration Statement on Form S-3, Registration No. 333-173268 (as amended or supplemented, the “Initial Registration Statement”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), and (ii) a second Registration Statement on Form S-3 filed pursuant to Rule 462(b) promulgated under the Securities Act (the “462(b) Registration Statement,” and together with the Initial Registration Statement, the “Registration Statement”). This opinion is furnished to you in connection with your filing of the 462(b) Registration Statement, which is registering the offering by Plug Power Inc., a Delaware corporation (the “Company”), of up to $3,734,329 of shares (the “Shares”) of the Company’s Common Stock, $0.01 par value per share (the “Common Stock”). The Shares are being sold to the underwriter named in, and pursuant to, an Underwriting Agreement between the Company and such underwriter (the “Underwriting Agreement”).

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

The opinions set forth below are limited to the Delaware General Corporation Law (which includes reported judicial decisions interpreting the Delaware General Corporation Law) and the law of New York.

Based on the foregoing, and subject to the additional qualifications set forth below, we are of the opinion that:

The Shares have been duly authorized and, when the price upon which the Shares are to be sold have been approved by or on behalf of the Board of Directors of the Company (or a duly authorized committee of the Board of Directors) and the Shares have been issued and delivered against payment in the manner described in the Registration Statement, the Shares will be validly issued, fully paid and non-assessable.

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Registration Statement and to the references to our firm under the caption “Legal Matters” in the Registration Statement. 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.

Very truly yours,

/s/ Goodwin Procter LLP

GOODWIN PROCTER LLP

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