Osler, Hoskin & Harcourt LLP

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July 13, 2021

Acasti Pharma Inc.

3009 boul. De la Concorde E.

Suite 102

Laval, Québec

H7E 2B5

Dear Sirs/Mesdames:

Re:  Registration Statement No. 333-257589; up to 170,500,000 common shares, no par value

We have acted as Canadian counsel to Acasti Pharma Inc. (the “Corporation”), a corporation governed by the Business Corporations Act (Québec), in connection with the proposed issuance of up to 170,500,000 common shares, no par value (the “Shares”). The Shares are included in a registration statement on Form S-4 under the Securities Act of 1933, as amended (the “Act”), initially filed with the Securities and Exchange Commission (the “Commission”) on June 30, 2021 (Registration No. 333-257589) (the “Registration Statement”), in connection with that certain Agreement and Plan of Merger (the “Merger Agreement”), dated as of May 7, 2021, by and among the Corporation, Acasti Pharma U.S., Inc. and Grace Therapeutics, Inc., which is included as Annex A to the Registration Statement. This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement, other than as expressly stated herein with respect to the issue of the Shares.

We have examined the Merger Agreement, the Registration Statement and all such corporate and public records, statutes and regulations and have made such investigations and have reviewed such other documents as we have deemed relevant and necessary and have considered such questions of law as we have considered relevant and necessary in order to give the opinion hereinafter set forth. As to various questions of fact material to such opinions which were not independently established, we have relied upon a certificate of an officer of the Corporation.

In reviewing the foregoing documents and in giving this opinion, we have assumed (a) the legal capacity of all individuals, the genuineness of all signatures, the veracity of the information contained therein, the authenticity of all documents submitted to us as originals and the conformity to authentic or original documents of all documents submitted to us as certified, conformed, electronic, photostatic or facsimile copies and (b) the completeness, truth and accuracy of all facts set forth in the official public records, certificates and documents supplied by public officials or otherwise conveyed to us by public officials.

We are qualified to practice law in the Province of Québec and this opinion is rendered solely with respect to the Province of Québec and the federal laws of Canada applicable in the Province of Québec.

Page  2

Where our opinion expressed herein refers to the Shares having been issued as being “fully-paid and non-assessable” common shares of the Corporation, such opinion assumes that all required consideration (in whatever form) has been paid or provided. No opinion is expressed as to the adequacy of any consideration received.

On the basis of the foregoing, we are of the opinion that, when the Shares will have been issued pursuant to and in accordance with the terms of the Merger Agreement, the Shares will be validly issued, fully paid and non-assessable.

In rendering the foregoing opinion, we have assumed that prior to the delivery of any Shares, the Registration Statement will have been declared effective under the Act and that the registration will apply to all of the Shares and will not have been modified or rescinded and that there will not have occurred any change in law affecting the validity of the issuance of such Shares.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Registration Statement and to the reference to our firm 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 Act or the rules and regulations of the Commission thereunder.

Yours very truly,

/s/ Osler, Hoskin & Harcourt LLP

Osler, Hoskin & Harcourt LLP