HOGAN & HARTSON
L.L.P.

111 SOUTH CALVERT STREET
BALTIMORE, MD 21202
TEL (410) 659-2700
FAX (410) 539-6981
WWW.HHLAW.COM

November 23, 2005

Board of Directors

Under Armour, Inc.

Tide Point

1020 Hull Street, 3rd floor

Baltimore, Maryland 21230

Ladies and Gentlemen:

We are acting as counsel to Under Armour, Inc., a Maryland corporation (the “Company”), in connection with its registration statement on Form S-8, as amended (the “Registration Statement”), filed with the Securities and Exchange Commission relating to the proposed public offering of up to 2,700,000 shares of the Company’s Class A Common Stock, par value $.0003^1/3 per share, all of which shares (the “Shares”) are reserved for issuance pursuant to the Company’s 2005 Omnibus Long-Term Incentive Plan (the “Plan”). This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R. § 229.601(b)(5), in connection with the Registration Statement.

For purposes of this opinion letter, we have examined copies of the following documents:

1. An executed copy of the Registration Statement.

WASHINGTON, DC

BALTIMORE BEIJING BERLIN BOULDER BRUSSELS BUDAPEST CARACAS COLORADO SPRINGS DENVER GENEVA HONG KONG LONDON

LOS ANGELES MIAMI MOSCOW MUNICH NEW YORK NORTHERN VIRGINIA PARIS SHANGHAI TOKYO WARSAW

Board of Directors

Under Armour, Inc.

November 23, 2005

Page 2

2. The Charter of the Company, as certified by the Maryland State Department of Assessments and Taxation on November 17, 2005, and by the Secretary of the Company on the date hereof as being complete, accurate, and in effect.

3. The Amended and Restated Bylaws of the Company, as certified by the Secretary of the Company on the date hereof as being complete, accurate, and in effect.

4. The Plan as certified on the date hereof by the Secretary of the Company as having been adopted by the Board of Directors of the Company and being complete, accurate and in effect.

5. Resolutions of the Board of Directors of the Company and the Compensation Committee of the Board of Directors relating to, among other things, approval of the Plan and wards thereunder, as certified by the Secretary of the Company on the date hereof as being complete, accurate and in effect.

6. Resolutions of the Stockholders of the Company relating to, among other things, approval of the Plan, as certified by the Secretary of the Company on the date hereof as being complete, accurate and in effect.

In our examination of the aforesaid documents, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the accuracy and completeness of all documents submitted to us, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to us as copies (including telecopies). This opinion letter is given, and all statements herein are made, in the context of the foregoing.

This opinion letter is based as to matters of law solely on the Maryland General Corporation Law, as amended. We express no opinion herein as to any other laws, statutes, ordinances, rules, or regulations. As used herein,

Board of Directors

Under Armour, Inc.

November 23, 2005

Page 3

the term “Maryland General Corporation Law, as amended” includes the statutory provisions contained therein, all applicable provisions of the Maryland Constitution and reported judicial decisions interpreting these laws.

Based upon, subject to and limited by the foregoing, we are of the opinion that when issued and delivered in connection with the Registration Statement and the Plan (with the Company having received the consideration therefor, the form of which is in accordance with applicable law), the Shares will be validly issued, fully paid, and nonassessable.

This opinion letter has been prepared for your use in connection with the Registration Statement and speaks as of the date hereof. We assume no obligation to advise you of any changes in the foregoing subsequent to the delivery of this opinion letter.

We hereby consent to the filing of this opinion letter as Exhibit 5.01 to the Registration Statement and to the reference to this firm under the caption “Legal Matters” in the prospectus constituting a part of the Registration Statement. In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Securities Act of 1933, as amended.

Very truly yours,

/s/ HOGAN & HARTSON L.L.P.

HOGAN & HARTSON L.L.P.