Exhibit 5.1

June 20, 2024

Biogen Inc.

Registration Statement on Form S-8

Ladies and Gentlemen:

We have acted as counsel for Biogen Inc., a Delaware corporation (the “Company”), in connection with the registration statement on Form S-8 (the “Registration Statement”), filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to the 11,877,397 shares of the Company’s common stock, par value $0.0005 per share (the “Shares”), issuable pursuant to the Company’s (a) 2024 Omnibus Equity Plan (the “2024 Plan”) and (b) 2024 Employee Share Purchase Plan (together with the 2024 Plan, the “Plans”).

In that connection, we have examined originals, or copies certified or otherwise identified to our satisfaction, of such documents, corporate records and other instruments as we have deemed necessary or appropriate for the purposes of this opinion, including, without limitation: (a) the Amended and Restated Certificate of Incorporation of the Company, as amended; (b) the Amended and Restated Bylaws of the Company; (c) certain resolutions adopted by the Board of Directors of the Company and (d) the Plans.

In rendering our opinion, we have assumed, with your consent and without independent investigation or verification, the genuineness of all signatures, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals and the conformity to authentic original documents of all documents submitted to us as duplicates or copies. We have relied, with respect to factual matters, upon certificates or comparable documents of officers and representatives of the Company.

Based on the foregoing and in reliance thereon, and subject to compliance with applicable state securities laws, we are of the opinion that the Shares when, and if, issued pursuant to the terms of the Plans will be validly issued, fully paid and non-assessable.

We are admitted to practice in the State of New York, and we express no opinion as to matters governed by any laws other than the General Corporation Law of the State of Delaware and the Federal laws of the United States of America.

We hereby consent to the filing of this opinion with the Commission as Exhibit 5.1 to the Registration Statement. In giving this consent, we do not hereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

Very truly yours,

/s/ Cravath, Swaine & Moore LLP

Biogen Inc.

225 Binney Street

Cambridge, Massachusetts 02142

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