Exhibit 5.1

  Seaport West 155 Seaport Boulevard Boston, MA 02210-2600
617 832 1000 main 617 832 7000 fax

June 16, 2021

Cyclerion Therapeutics, Inc.

245 First Street, 18th Floor

Cambridge, Massachusetts 02142

Re:          Registration Statement on Form S-3

Ladies and Gentlemen:

We have acted as special Massachusetts counsel to Cyclerion Therapeutics, Inc., a Massachusetts corporation (the “Company”), in connection with its filing of a registration statement on Form S-3 (the “Registration Statement”) with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), on the date hereof.

The Registration Statement relates to the offer and sale of 5,735,988 shares (the “Shares”) of the Company’s common stock, no par value per share, by stockholders of the Company identified in the Registration Statement.

We have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter. We have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters. In our examination, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals and the conformity to authentic original documents of all documents submitted to us as copies.

The opinions expressed below are limited to the internal laws of the Commonwealth of Massachusetts.

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof, the Shares have been validly issued and are fully paid and nonassessable.

Cyclerion Therapeutics, Inc.

June 16, 2021

Page 2

This opinion is to be relied upon only in connection with the offer and sale of the Shares while the Registration Statement and any and all required post-effective amendments thereto are effective.

This opinion is being delivered solely for the benefit of the Company and such other persons as are entitled to rely upon it pursuant to applicable provisions of the Securities Act.  This opinion may not be used, quoted, relied upon or referred to for any other purpose, nor may this opinion be used, quoted, relied upon or referred to by any other person, for any purpose, in each case without our prior written consent.

We consent to your filing this opinion as an exhibit to the Registration Statement and to the reference to our firm in the Prospectus under the heading “Legal Matters.” In giving such consent, we do not thereby 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 of the Commission thereunder.

Very truly yours,
FOLEY HOAG llp
By:	/s/ John D. Hancock
	a Partner