Exhibit 5.1

June 19, 2017

Byline Bancorp, Inc.,

180 North LaSalle Street, Suite 300,

Chicago, Illinois 60601.

Ladies and Gentlemen:

In connection with the registration under the Securities Act of 1933 (the “Act”) of 6,555,000 shares (the “Securities”) of Common Stock, par value $0.01 per share, of Byline Bancorp, Inc., a Delaware corporation (the “Company”), of which up to 4,630,194 shares will be issued and sold by the Company (the “Primary Shares”) and 1,924,806 shares will be sold by selling stockholders (the “Secondary Shares”), we, as your counsel, have examined such corporate records, certificates and other documents, and such questions of law, as we have considered necessary or appropriate for the purposes of this opinion. Upon the basis of such examination, it is our opinion that (i) when the registration statement relating to the Primary Shares (the “Registration Statement”) has become effective under the Act, the terms of the sale of the Primary Shares have been duly established in conformity with the Company’s amended and restated certificate of incorporation, and the Primary Shares have been duly issued and sold as contemplated by the Registration Statement, the Primary Shares will be validly issued, fully paid and nonassessable and (ii) the Secondary Shares have been validly issued and are fully paid and nonassessable.

In rendering the foregoing opinion, we are not passing upon, and assume no responsibility for, any disclosure in any registration statement or any related prospectus or other offering material relating to the offer and sale of the Securities.

The foregoing opinion is limited to the Federal laws of the United States and the General Corporation Law of the State of Delaware, and we are expressing no opinion as to the effect of the laws of any other jurisdiction.

We have relied as to certain factual matters on information obtained from public officials, officers of the Company and other sources believed by us to be responsible.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to us under the heading “Validity of Common Stock” in the Prospectus. 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.