Exhibit 5.1

[Letterhead of Wachtell, Lipton, Rosen & Katz]

May 29, 2020

Regeneron Pharmaceuticals, Inc.

777 Old Saw Mill River Road

Tarrytown, New York 10591

Ladies and Gentlemen:

We have acted as special counsel to Regeneron Pharmaceuticals, Inc., a New York corporation (the “Company”), in connection with the offer and sale by Sanofi and Aventisub LLC (the “Selling Shareholders”) of shares of common stock, par value $0.001 per share, of the Company (the “Securities”) in an underwritten public offering pursuant to the Underwriting Agreement, dated as of May 26, 2020, by and among the Company, the Selling Shareholders and BofA Securities, Inc. and Goldman Sachs & Co. LLC, as representatives of the several underwriters named therein.

We have examined: (i) the Registration Statement on Form S-3ASR (Registration No. 333-228352) initially filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), on November 13, 2018; (ii) the preliminary prospectus dated May 26, 2020, in the form filed with the Commission pursuant to Rule 424(b) under the Securities Act on May 26, 2020 (the “Preliminary Prospectus”); (iii) the final prospectus dated May 26, 2020, in the form filed with the Commission pursuant to Rule 424(b) under the Securities Act on May 28, 2020 (the “Final Prospectus”, and together with the Preliminary Prospectus, the “Prospectus”) in connection with the offering and sale by the Company of the Securities; (iv) the Restated Certificate of Incorporation of the Company, as amended, as in effect on the date hereof, and the Amended and Restated By-Laws of the Company as in effect on the date hereof; and (v) such other corporate records, certificates and other documents and such matters of law, in each case, as we have deemed necessary or appropriate.

In rendering this opinion, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to authentic original documents of all documents submitted to us as copies and the legal capacity of all individuals executing such documents. As to any facts material to this opinion that we did not independently establish or verify, we have, with your consent, relied upon the statements, certificates and representations of public officials and officers of the Company.

Regeneron Pharmaceuticals, Inc.

May 29, 2020

Page 2

Based upon the foregoing and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that the Securities are legally issued, fully paid and nonassessable.

We are members of the Bar of the State of New York. This opinion is limited to the laws of the State of New York and the federal securities laws of the United States, and we express no opinion as to the effect on the matters covered by this opinion of the laws of any other jurisdiction.

This opinion letter speaks only as of its date and is delivered in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act. We assume no obligation to advise the Company or any other person, or to make any investigations, as to any legal developments or factual matters arising subsequent to the date hereof that might affect the opinions expressed herein. We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Company’s Current Report on Form 8-K filed with the Commission on May 29, 2020, and its incorporation by reference into the Registration Statement. In addition, we consent to the reference to us under the caption “Validity of the Common Stock” in the Prospectus. In giving such consent, we do not thereby admit that we are an expert within the meaning of Section 7 of the Securities Act and the rules and regulations of the Commission promulgated thereunder.