Exhibit 5.1


Our File No.:



November 1, 2022

ProMIS Neurosciences Inc.

Suite 200, 1920 Yonge Street

Toronto, Ontario

M4S 3E2

Attention: Board of Directors


ProMIS Neurosciences Inc. (the Corporation)

Resale Registration Statement on Form S-1 (the Registration Statement)‎‎

We have acted as Ontario legal counsel to ProMIS Neurosciences Inc. (the Corporation) with respect to certain legal matters in connection with the registration statement on Form S-1 filed on the date hereof (the Registration Statement) with the Securities and Exchange Commission (the Commission), pursuant to the U.S Securities Act of 1933, as amended (the Securities Act), relating to the registration by the Corporation under the Securities Act of the resale or other disposition from time to time by the selling shareholders identified in the Registration Statement (the Selling Shareholders) of 1,729,694 common shares, no par value (the Common Shares), of the Corporation comprised of:


1,383,755 Common Shares (the Unit Shares)underlying 1,383,755 units (the Units) issued pursuant to unit purchase agreements entered into between the Corporation and Selling Shareholders, dated October 11, 2022 (the Unit Purchase Agreements), in connection with a private placement offering (the Unit Private Placement), at a price of US$5.40 per Unit; and


345,939 Common Shares of the Corporation issuable upon the exercise of warrants (the Warrant Shares) underlying the Units (each, a Warrant) issued pursuant to the Unit Purchase Agreements in connection with the Unit Private Placement, with each whole Warrant entitling the holder thereof to purchase one Warrant Share at a price of US$7.50 for a period of five years from the date of issue and may be exercised commencing six months from the date of issue.

The Unit Shares and Warrant Shares are being registered pursuant to a Registration Rights Agreement, dated October 11, 2022, entered into between the Corporation and Selling Shareholders (the Registration Rights Agreement), pursuant to which the Corporation is required to prepare and file a registration statement with the Commission under the

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November 1, 2022

Page 2

Securities Act, covering the resale of the Unit Shares and Warrant Shares issued to Selling Shareholders under the Unit Purchase Agreements.

Scope of Review and Reliances

For the purpose of rendering our opinions expressed below, we have examined and relied upon the following documents:


the form of Unit Purchase Agreement;


the Registration Rights Agreement;


the Registration Statement and the exhibits thereto;


the articles and bylaws of the Corporation in effect as of the date hereof (the Constating Documents);


the form of certificate representing the Warrants (the Warrant Certificate);


a certificate of the Chief Executive Officer and the Chief Financial Officer of the Corporation as to certain factual matters relating to the Offering dated the date hereof (the Officers Certificate); and


such other documents as we have deemed relevant.

We have relied exclusively upon the certificates, documents and records referred to above with respect to the accuracy of the factual matters contained therein and we have not performed any independent investigation or verification of such factual matters.

We have considered such questions of law, made such investigations and examined and relied upon such originals, facsimiles or copies, certified or otherwise identified to our satisfaction, of such additional public and corporate records, records of corporate proceedings, certificates and other documents as we have considered relevant or necessary in order to render our opinions expressed herein.

Our opinion expressed herein is subject in all respects to the following assumptions, limitations and qualifications:


We are solicitors qualified to practice law in the Province of Ontario. The opinion is limited to the laws of the Province of Ontario, including all applicable provisions of the Canada Business Corporations Act (the CBCA) and the federal laws of Canada applicable in Ontario, in each case as in effect on the date hereof. We have not considered, and have not expressed any opinion with regard to, or as to the effect of, any other law, rule or regulation, state or federal, applicable to the Corporation. In particular, we express no opinion as to United States federal securities laws. We have no responsibility and assume no responsibility to advise you of any changes to applicable laws or any changes involving the Company after the date hereof.


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We have assumed: (i) the genuineness of all signatures on documents examined by us, including electronic signatures; (ii) the legal capacity of the officers of the Corporation; (iii) the authenticity of all documents submitted to us as originals; (iv) the conformity to authentic originals of all documents submitted to us as certified, conformed, photostatic or other copies; and (v) that the documents, in the forms submitted to us for our review, have not been and will not be altered or amended in any respect.


We have assumed that each of the statements made and certified in the Officers Certificate was true and correct when made, has at no time since being made and certified become untrue or incorrect and remains true and correct on the date hereof.


We have assumed that the time of issuance of any Warrant Shares the Corporation will validly exist under the CBCA and will have the necessary corporate power for such issuance.


We have assumed the time of issuance of any Warrant Shares, the Constating Documents are in full force ‎‎and effect and have not been amended, restated, supplemented or otherwise altered in such a matter to prohibit such issuance and there has been no authorization of any such amendment, restatement, supplement or other alteration, in either case since the date hereof.


Based and relying on the foregoing, we are of the opinion that:


The 1,383,755 Unit Shares have been duly and validly issued as fully paid and non-assessable Common Shares in the capital of the Corporation.


The issuance of the 345,939 Warrant Shares upon due exercise of the Warrants have been duly authorized by the Corporation and upon exercise of such Warrants in accordance with the terms of the Warrant Certificates, including full payment of the exercise price per Warrant Share purchased, the Warrant Shares will be duly and validly issued as fully paid and non-assessable Common Shares in the capital of the Corporation.

Our opinions are rendered solely for the benefit of the addressees, are being delivered in connection with the transactions described herein, and may not be relied upon by any other person or in connection with any other transaction, quoted from or referred to in any other documents, or furnished (either in its original form or by copy) to any other person without our prior written consent.

We consent to the use of this opinion as an exhibit to the Registration Statement and further consent to the use of our name wherever appearing in the Registration Statement and in any amendment thereto.

Yours Truly,

/s/ McMillan LLP