Exhibit 5.1

October 15, 2014

Anchor BanCorp Wisconsin Inc.

25 West Main Street

Madison, Wisconsin 53703

Ladies and Gentlemen:

We have acted as special counsel to Anchor BanCorp Wisconsin Inc., a Delaware corporation (the “Company”), in connection with the initial public offering by the Company (the “IPO”) relating to the sale (a) by the Company to the Underwriters (as defined below) of 250,000 shares (the “Firm Shares”) of the Company’s common stock, par value $0.01 per share (the “Common Stock”), (b) by the Selling Stockholders (as defined in the Underwriting Agreement) to you of 121,959 shares of Common Stock (the “Secondary Shares”) and (c) up to an additional 55,794 shares of Common Stock (the “Option Shares” and, together with the Firm Shares, the “Company Shares”) at the Underwriters’ option to cover over-allotments.

This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K of the General Rules and Regulations under the Securities Act of 1933, as amended (the “Act”).

In connection with this opinion, we have examined originals or copies, certified or otherwise identified to our satisfaction, of (a) the Registration Statement on Form S-1 (File No. 333-192964) of the Company, as filed with the Securities and Exchange Commission (the “Commission”) under the Act on December 19, 2013; (b) Pre-Effective Amendments Nos. 1, 2, 3, 4, 5, 6 and 7 thereto (such Registration Statement, as so amended, being hereinafter referred to as the “Registration Statement”); (c) the form of underwriting agreement (the “Underwriting Agreement”) proposed to be entered into by and among Sandler O’Neill & Partners, L.P., as representative of the several underwriters named in Schedule I thereto (the “Underwriters”), the Company and the Selling Stockholders, filed as Exhibit 1.1 to the Registration Statement; (d) the Amended and Restated Certificate of Incorporation of the Company, as amended to date and currently in effect; (e) the Amended and Restated Bylaws of the Company, as amended to date and currently in effect; and (f) certain resolutions of the Board of Directors of the Company relating to the issuance of the Company Shares and related matters. We have also examined

Anchor BanCorp Wisconsin Inc.

October 15, 2014

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originals or copies, certified or otherwise identified to our satisfaction, of such records of the Company and such agreements, certificates and receipts of public officials, certificates of officers or other representatives of the Company and others, and such other documents, certificates and records as we have deemed necessary or appropriate as a basis for the opinions set forth below.

We have assumed the legal capacity of all natural persons, the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as facsimile, electronic, certified or photostatic copies, and the authenticity of the originals of such copies. In making our examination of executed documents, we have assumed that the parties thereto, other than the Company, had the power, corporate or other, to enter into and perform all obligations thereunder and have also assumed the due authorization by all requisite action, corporate or other, and the execution and delivery by such parties of such documents and the validity and binding effect thereof on such parties. As to any facts material to the opinions expressed herein that we did not independently establish or verify, we have relied upon statements and representations of officers and other representatives of the Company and others and of public officials.

Members of our firm are admitted to the bar in the State of New York, and we do not express any opinion with respect to the law of any jurisdiction other than Delaware corporate law (including, to the extent applicable, the Delaware constitution and judicial decisions) and we do not express any opinion as to the effect of any other laws on the opinions herein stated.

Based upon and subject to the foregoing, we are of the opinion that:

1. Upon the (i) due action by the Board of Directors of the Company to determine the price per share of the Company Shares and (ii) due execution and delivery of the Underwriting Agreement and issuance of the Company Shares against payment therefor in accordance with the Underwriting Agreement, the Company Shares will have been duly authorized by all necessary corporate action of the Company and will be validly issued, fully paid and nonassessable.

2. The Secondary Shares have been duly authorized and validly issued and are fully paid and non-assessable.

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

Very truly yours,

/s/ Skadden, Arps, Slate, Meagher & Flom LLP