Exhibit 5.1

LOGO

Davis Polk & Wardwell LLP

450 Lexington Avenue
New York, NY 10017

davispolk.com

February 4, 2026

Clear Street Group Inc.

150 Greenwich Street, 45^th Floor

New York, New York 10007

Ladies and Gentlemen:

Clear Street Group Inc., a Delaware corporation (the “Company”), has filed with the Securities and Exchange Commission a Registration Statement on Form S-1 (the “Registration Statement”) and the related prospectus (the “Prospectus”) for the purpose of registering under the Securities Act of 1933, as amended (the “Securities Act”), 27,380,952 shares of its Class A common stock, par value $0.00001 per share (the “Securities”), including 3,571,428 shares subject to the underwriters’ option to purchase additional shares, as described in the Registration Statement.

We, as your counsel, have examined originals or copies of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary or advisable for the purpose of rendering this opinion.

In rendering the opinion expressed herein, we have, without independent inquiry or investigation, assumed that (i) all documents submitted to us as originals are authentic and complete, (ii) all documents submitted to us as copies conform to authentic, complete originals, (iii) all signatures on all documents that we reviewed are genuine, (iv) all natural persons executing documents had and have the legal capacity to do so, (v) all statements in certificates of public officials and officers of the Company that we reviewed were and are accurate and (vi) all representations made by the Company as to matters of fact in the documents that we reviewed were and are accurate.

Based upon the foregoing, we advise you that, in our opinion, when the price at which the Securities to be sold has been approved by or on behalf of the Board of Directors of the Company and when the Securities have been issued and delivered against payment therefor in accordance with the terms of the Underwriting Agreement referred to in the Prospectus which is a part of the Registration Statement, the Securities will be validly issued, fully paid and non-assessable.

We are members of the Bar of the State of New York and the foregoing opinion is limited to the laws of the State of New York and the General Corporation Law of the State of Delaware.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and further consent to the reference to our name under the caption “Legal Matters” in the Prospectus. In giving this consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.

Very truly yours,

/s/ Davis Polk & Wardwell LLP

Clear Street Group Inc. S-1/A