Exhibit 5.1

SULLIVAN & CROMWELL LLP

125 BROAD STREET

NEW YORK, NEW YORK 10004

March 10, 2020                        

Bank of Montreal,

    100 King Street West,

        1 First Canadian Place,

            Toronto, Ontario,

                Canada M5X 1A1.

Ladies and Gentlemen:

We are acting as counsel to Bank of Montreal, a Canadian chartered bank (the “Bank”), in connection with the issuance and delivery of US$1,500,000,000 principal amount of the Bank’s Floating Rate Notes due 2023 (the “Notes”). The Bank filed with the Securities and Exchange Commission, on April 7, 2017, a registration statement on Form F-3 (File No. 333-217200), as subsequently amended on April 27, 2017 (the “Registration Statement”), under the Securities Act of 1933 (the “Act”) relating to, among other things, the proposed offer and sale of up to US$25,000,000,000 aggregate amount of securities of the Bank, including the Notes. The Notes are being issued under an indenture, dated as of January 25, 2010, as supplemented by the First Supplemental Indenture thereto, dated as of September 23, 2018 (together, the “Indenture”), between the Bank and Wells Fargo Bank, National Association, as trustee (the “Trustee”).

We have examined such corporate records, certificates and other documents, and such questions of United States federal and New York state law, as we have considered necessary or appropriate for the purposes of this opinion. Upon the basis of such examination, we advise you that, in our opinion, the Notes constitute valid and legally binding obligations of the Bank, subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and similar laws of general applicability relating to or affecting creditors’ rights and to general equity principles.

The foregoing opinion is limited to the federal laws of the United States and the laws of the State of New York, and we are expressing no opinion as to the effect of the laws of any other jurisdiction.

In rendering the foregoing opinion, we are not passing upon, and assume no responsibility for, any disclosure in the Registration Statement or any related prospectus or other offering material regarding the Bank or the Notes or their offering and sale.

We have relied as to certain factual matters on information obtained from public officials, officers of the Bank and other sources believed by us to be responsible, and we have assumed, without independent verification, that the Bank is duly organized, validly existing and in good standing under the laws of Canada, that all corporate action by the Bank related to the Notes was duly authorized as a matter of Canadian law, that the Indenture has been duly authorized, executed and delivered by the Bank insofar as the laws of Canada are concerned, that the Indenture has been duly authorized, executed and delivered by the Trustee, that the Notes conform to the specimens thereof examined by us, that the Notes have been duly authenticated by one of the Trustee’s authorized officers, that the Notes have been delivered against payment as contemplated in the Registration Statement and that the signatures on all documents examined by us are genuine.

We hereby consent to the filing of this opinion as an exhibit to a Current Report on Form 6-K to be incorporated by reference in the Registration Statement. In giving this consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act.

Exhibit 5.2

Osler, Hoskin & Harcourt LLP Box 50, 1 First Canadian Place Toronto, Ontario, Canada M5X 1B8 416.362.2111 MAIN 416.862.6666 FACSIMILE

Dear Sirs/Mesdames:

Bank of Montreal – US$1,500,000,000 Floating Rate Notes due 2023

We have acted as Canadian counsel to Bank of Montreal (the “Bank”) in connection with the issue and sale today (the “Offering”) by the Bank of US$1,500,000,000 aggregate principal amount of the Bank’s Floating Rate Notes due 2023 (the “Notes”), constituting part of the Senior Medium-Term Notes, Series E of the Bank, pursuant to the distribution agreement dated September 23, 2018 (the “Distribution Agreement”) between the Bank and BMO Capital Markets Corp., the terms agreement dated March 5, 2020 (the “Terms Agreement”) between the Bank and BMO Capital Markets Corp. and Deutsche Bank Securities Inc., and the senior indenture dated as of January 25, 2010, as it may be amended and supplemented from time to time, including as supplemented by the First Supplemental Indenture thereto dated September 23, 2018 (collectively, the “Indenture”), between the Bank and Wells Fargo Bank, National Association, as trustee. Capitalized terms used herein and not otherwise defined herein have the meanings ascribed to such terms in the Terms Agreement.

We have examined copies, certified or otherwise authenticated to our satisfaction, of the following:

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We understand that the Registration Statement and the Prospectus were filed with the United States Securities and Exchange Commission in connection with the Notes.

For the purposes of our opinion below, we have examined such statutes, public and corporate records, certificates and other documents, and considered such questions of law, as we have considered relevant and necessary as a basis for the opinions hereinafter set forth. In such examination we have assumed the genuineness of all signatures and the authenticity of all documents submitted to us as certified or photostatic copies or facsimiles. For the purposes of the opinions expressed herein, we have, without independent investigation or verification, assumed that each of the Distribution Agreement, the Terms Agreement and the Indenture has been duly authorized, executed and delivered by, and constitutes or will constitute, as the case may be, a legal, valid and binding obligation of, each party thereto other than the Bank.

In giving this opinion, we express no opinion as to any laws other than the laws, at the date hereof, of the Province of Ontario and the federal laws of Canada applicable therein.

With respect to the continuing existence of the Bank as a Schedule I bank under the Bank Act (Canada) referred to in paragraph 1 below, we have relied, without independent investigation or verification, exclusively upon a Certificate of Confirmation dated March 9, 2020 issued by the Office of the Superintendent of Financial Institutions.

Based and relying upon and subject to the qualifications set forth herein, we are of the opinion that:

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The opinion set forth in paragraph 3 above as to the enforceability of the Indenture is subject to the qualifications that:

This opinion is rendered solely in connection with the transactions covered hereby, is limited to the matters stated herein, and no opinions may be implied or inferred beyond matters expressly stated herein.

We hereby consent to the filing of this opinion as an exhibit to a Current Report on Form 6-K to be incorporated by reference in the Registration Statement. In giving this consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the United States Securities Act of 1933.