Exhibit 5.1

October 20, 2026

T3 Defense Inc.

575 Fifth Ave., 14th Floor

New York, NY 10017

Ladies and Gentlemen:

We have acted as counsel to T3 Defense Inc. (formerly known as Nukkleus Inc.), a Delaware corporation (the “Company”), in connection with the preparation of the Company’s registration statement on Form S-1 (File No. 333-298751) (the “Registration Statement”), under the Securities Act of 1933, as amended (the “Securities Act”). The Registration Statement relates to the offering for resale by the selling stockholder listed therein of up to an aggregate of 10,000,000 shares (the “Shares”) of the Company’s common stock, par value $0.0001 per share, pursuant to a Securities Purchase Agreement, dated September 19, 2025, by and between the Company and such selling stockholder (the “Securities Purchase Agreement”).

In rendering the opinion set forth herein, we have examined the originals, or photostatic or certified copies, of (i) the Amended and Restated Certificate of Incorporation and Bylaws of the Company, each as amended to date, (ii) certain resolutions of the Board of Directors of the Company related to the filing of the Registration Statement, the authorization of the Securities Purchase Agreement and the authorization and issuance of the Shares and related matters, (iii) the Securities Purchase Agreement, (iv) the Registration Statement and all exhibits thereto and (v) such other records, documents and instruments as we deemed relevant and necessary for purposes of the opinion stated herein.

In making the foregoing examination, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as photostatic or certified copies, and the authenticity of the originals of such copies. As to all questions of fact material to this opinion, where such facts have not been independently established, we have relied, to the extent we have deemed reasonably appropriate, upon representations or certificates of officers of the Company or governmental officials.

We do not express any opinion herein concerning any law other than the General Corporation Law of the State of Delaware as in effect on the date hereof.

Based upon the foregoing, and subject to the qualifications, assumptions, limitations and exceptions stated herein, we are of the opinion that the Shares, when issued by the Company in accordance with the terms of the Securities Purchase Agreement, will be duly authorized, legally issued, fully paid and nonassessable.

This opinion speaks only as of the date hereof. We expressly disclaim any responsibility to advise you of any development or circumstance of any kind, including any change of law or fact, that may occur after the date of this opinion that might affect the opinion expressed herein.

One Vanderbilt Avenue   New York NY 10017-3852   Tel +1 212 547 5400   Fax +1 212 547 5444   US practice conducted through McDermott Will & Schulte LLP.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement. We hereby also consent to the reference to our firm under the caption “Legal Matters” in the Registration Statement. We do not admit in providing such consent that we are included within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.