Filed Pursuant to Rule 424(b)(3)

Registration No. 333-194745

PROSPECTUS SUPPLEMENT NO. 5 DATED JUNE 23, 2015

TO

PROSPECTUS DATED MAY 22, 2015

(AS SUPPLEMENTED)

ARCH THERAPEUTICS, INC.

PROSPECTUS

Up to 31,279,926 Shares of Common Stock

This Prospectus Supplement No. 5 supplements the prospectus of Arch Therapeutics, Inc. (“the “Company”, “we”, “us”, or “our”) dated May 22, 2015 (as supplemented to date, the “Prospectus”) with the following attached document which we filed with the Securities and Exchange Commission on June 23, 2015:

This Prospectus Supplement No. 5 should be read in conjunction with the Prospectus, which is required to be delivered with this Prospectus Supplement. This prospectus supplement updates, amends and supplements the information included in the Prospectus. If there is any inconsistency between the information in the Prospectus and this prospectus supplement, you should rely on the information in this prospectus supplement.

This prospectus supplement is not complete without, and may not be delivered or utilized except in connection with, the Prospectus, including any amendments or supplements to it.

Investing in our common stock involves a high degree of risk. Before making any investment in our common stock, you should carefully consider the risk factors for our common stock, which are described in the Prospectus, as amended or supplemented.

You should rely only on the information contained in the Prospectus, as supplemented or amended by this Prospectus Supplement No. 5 and any other prospectus supplement or amendment thereto. We have not authorized anyone to provide you with different information.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this Prospectus Supplement No. 5 is June 23, 2015

INDEX TO FILINGS

ANNEX A

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 OR 15(d) of The Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): June 22, 2015

ARCH THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (617) 431-2313

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Item 1.01 Entry into a Material Definitive Agreement.

On June 22, 2015 (the “Amendment Date”), the Company entered into that certain Amendment to Series A Warrants and Series C Warrants to Purchase Common Stock (the “Amendment”), dated as of the Amendment Date, with Cranshire Capital Master Fund, Ltd. (“Cranshire”), to (i) delete the full ratchet anti-dilution provisions set forth in the Series A Warrants and Series C Warrants (collectively, the “2014 Private Placement Warrants”); and (ii) extend the expiration date of the Series C Warrants from to 5:00 p.m., New York time, on July 2, 2015 to 5:00 p.m., New York time, on July 2, 2016. In consideration of Cranshire’s entrance into the Amendment (and for no additional consideration), the Company agreed to issue to the holders of the 2014 Private Placement Warrants up to 570,000 shares of Company’s common stock (the “Inducement Shares”) conditioned, in the case of each such holder, upon the execution and delivery by each such holder of an investor certificate to the Company.

The issuance of the Inducement Shares to the holders of the 2014 Private Placement Warrants who execute and deliver their respective investor certificates to the Company (each, an “Eligible Warrantholder”) has not been registered under the Securities Act of 1933, as amended (the “Securities Act”), and the Inducement Shares may not be offered or sold in the United States absent registration under or exemption from the Securities Act and any applicable state securities laws. The Inducement Shares will be issued in reliance upon an exemption from registration afforded by Section 4(a)(2) of the Securities Act based on the following facts: each Eligible Warrantholder will have represented, among other things, that it is an accredited investor as defined in Rule 501 promulgated under the Securities Act, that it is acquiring the Inducement Shares for investment only and not with a view towards, or for resale in connection with, the public sale or distribution thereof in violation of applicable securities laws; that it has sufficient knowledge of the Company to reach an informed and knowledgeable decision to acquire the Inducement Shares; the Company used no advertising or general solicitation in connection with the issuance of the Inducement Shares to the Investors; and the Inducement Shares will be issued as restricted securities. This Current Report on Form 8-K is not and shall not be deemed to be an offer to sell or the solicitation of an offer to buy any of the Inducement Shares.

The preceding description of the Amendment is qualified in its entirety by reference to the copy of the Amendment filed herewith as Exhibit 10.1 to this Current Report on Form 8-K, which is incorporated herein by reference.

Item 3.02 Unregistered Sales of Equity Securities.

Reference is made to the disclosure set forth in Item 1.01 of this Current Report on Form 8-K, which disclosure is incorporated by reference into this Item 3.02.

Item 8.01 Other Events.

In accordance with Section 8 of the Series A Warrants and Series C Warrants, on June 23, 2015, the Company distributed the notice attached hereto as Exhibit 99.1 to this Current Report on Form 8-K to the Holders of the Series A Warrants and Series C Warrants.

Item 9.01 Financial Statements and Exhibit

(d) Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Exhibit List

Exhibit 10.1

AMENDMENT TO SERIES A WARRANTS AND SERIES C WARRANTS TO PURCHASE COMMON STOCK

This Amendment to Series A Warrants and Series C Warrants to Purchase Common Stock (the “Amendment”) by and between Arch Therapeutics, Inc., a Nevada corporation (the “Company”), and Cranshire Capital Master Fund, Ltd. (“Cranshire”) is made as of June 22, 2015 (the “Effective Date”).

RECITALS

WHEREAS, on January 30, 2014, the Company entered into that certain Securities Purchase Agreement (the “Securities Purchase Agreement”) with the investors named on the Schedule of Buyers attached thereto (collectively, the “Holders”) pursuant to which, among other things, the Company issued to the Holders shares of Common Stock (as defined in the Series A Warrants and Series C Warrants (each as defined in the Securities Purchase Agreement)) and the Series A Warrants and Series C Warrants;

WHEREAS, Section 2(b) of the Series A Warrants and Series C Warrants provides that upon certain issuances or sales or deemed issuances or sales of shares of Common Stock for a consideration per share (the “New Issuance Price”) at less than a price equal to the Exercise Price in effect immediately prior to such issuance or sale or deemed issuance or sale (a “Dilutive Issuance”), the Exercise Price in effect immediately prior to such Dilutive Issuance shall be reduced to an amount equal to the New Issuance Price (the “Anti-Dilution Adjustment Provision”);

WHEREAS, the Expiration Date of the Series C Warrants is 5:00 p.m., New York time, on July 2, 2015;

WHEREAS, Section 9 of the Series A Warrants and Series C Warrants provides that the provisions of the Series A Warrants and Series C Warrants may be amended only with the written consent of the Company and the Significant Buyers (as defined in the Securities Purchase Agreement);

WHEREAS, Cranshire is the only Significant Buyer;

WHEREAS, the Company and Cranshire, in its capacity as the only Significant Buyer, now wish to amend (i) the Series A Warrants and Series C Warrants to terminate the Anti-Dilution Adjustment Provision; and (ii) the Series C Warrants to extend the Expiration Date, in each case on the terms and conditions set forth herein (collectively, the “Warrant Amendments”); and

WHEREAS, in consideration for Cranshire entering into the Warrant Amendments, the Company has agreed to offer to issue to the Holders or their successors and assigns, as applicable, the Inducement Shares (as defined below), on the terms and conditions set forth herein.

NOW, THEREFORE, in consideration of the foregoing and for good and valuable consideration, the receipt and sufficiency which are hereby acknowledged, the Parties hereby agree as follows:

“(b)     [Reserved]”

“(b)     [Reserved]”

“(h)     “Expiration Date” means 5:00 p.m., New York time, on July 2, 2016.”

[signature page follows]

IN WITNESS WHEREOF, Cranshire and the Company have caused their respective signature page to this Amendment to be duly executed as of the date first written above.

Exhibit A

Form of Investor Certificate

Arch Therapeutics, Inc.

235 Walnut St., Suite 6

Framingham, MA 01702

Ladies and Gentlemen:

Reference is made to that certain Amendment to Series A Warrants and Series C Warrants to Purchase Common Stock (the “Amendment”) by and between Arch Therapeutics, Inc., a Nevada corporation (the “Company”), and Cranshire Capital Master Fund, Ltd. dated as of June 22, 2015 (the “Amendment”). Capitalized terms not otherwise defined in this Investor Certificate shall have the meaning given to them in the Amendment.

As a condition to receiving the undersigned’s allocation of the Inducement Shares in accordance with the Amendment and as set forth on Exhibit B thereto (the “Allocation Schedule”), the undersigned hereby represents and warrants to the Company as follows:

THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR APPLICABLE STATE SECURITIES LAWS. THE SECURITIES MAY NOT BE OFFERED FOR SALE, SOLD, TRANSFERRED OR ASSIGNED (I) IN THE ABSENCE OF (A) AN EFFECTIVE REGISTRATION STATEMENT FOR THE SECURITIES UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR (B) AN OPINION OF COUNSEL TO THE HOLDER (IF REQUESTED BY THE COMPANY), IN A FORM REASONABLY ACCEPTABLE TO THE COMPANY, THAT REGISTRATION IS NOT REQUIRED UNDER SAID ACT OR (II) UNLESS SOLD OR ELIGIBLE TO BE SOLD PURSUANT TO RULE 144 OR RULE 144A UNDER SAID ACT. NOTWITHSTANDING THE FOREGOING, THE SECURITIES MAY BE PLEDGED IN CONNECTION WITH A BONA FIDE MARGIN ACCOUNT OR OTHER LOAN OR FINANCING ARRANGEMENT SECURED BY THE SECURITIES.

[signature page follows]

IN WITNESS WHEREOF, the undersigned has caused this Investor Certificate to be duly executed as of the date set forth below.

Exhibit B

Allocation of Inducement Shares

* Assignee of Equitec Specialists, LLC

** Assignee of 0903746 B.C. Ltd

Exhibit 99.1

Arch Therapeutics, Inc.

235 Walnut Street, Suite 6

Framingham, Massachusetts

June 23, 2015

VIA E-MAIL

To whom it may concern:

You are receiving this notice (this “Notice”) in your capacity as a registered Holder (a “Holder”, and all Holders, collectively, the “Holders”) of the Series A Warrants and/or Series C Warrants (each as defined in the SPA (as defined below) and together, the “Warrants”) of Arch Therapeutics, Inc., a Nevada corporation (the “Company”), that were issued in connection with that certain Securities Purchase Agreement, dated as of January 30, 2014, among the Company and the investors listed on the Schedule of Buyers thereto (the “SPA”). Capitalized terms not otherwise defined in this Notice shall have the meaning given to them in the Series A Warrants and Series C Warrants.

Pursuant to Section 8 of the Series A Warrants and Series C Warrants, the Company hereby provides notice that June 22, 2015 (the “Amendment Execution Date”), the Company entered into that certain Amendment to Series A Warrants and Series C Warrants to Purchase Common Stock, dated as of the Amendment Execution Date, with Cranshire Capital Master Fund, Ltd. (the “Warrant Amendments”). A copy of the Warrant Amendments is attached hereto as Exhibits A.

Under the terms and conditions of the Warrant Amendments, the (i) anti-dilution provisions set forth in Section 2(b) of the Series A Warrants and Series C Warrants were terminated effective as of the Amendment Execution Date; (ii) the expiration date of the Series C Warrants was extended from 5:00 p.m., New York time, on July 2, 2015 to 5:00 p.m., New York time, on July 2, 2016; and (iii) subject to each Holder’s execution and delivery to the Company of an investor certificate substantially in the form attached as Exhibit A to the Warrant Amendments (an “Investor Certificate”), the Company shall issue to the Holders an aggregate total of 570,000 shares of Common Stock (the “Inducement Shares”) in direct proportion to the amount invested by each Holder (or such Holder’s predecessor or assignor, as applicable) under the SPA, and as set forth in Exhibit B to the Warrant Amendments.

To receive your allocation of the Inducement Shares, please complete and deliver to the Company a completed Investor Certificate at your earliest convenience.

[Signature Page Follows]

SCHEDULE I

Holders

Cranshire Capital Master Fund, Ltd.

c/o Cranshire Capital Advisors, LLC

3100 Dundee Road, Suite 703

Northbrook, IL 60062

Attn: Mitchell P. Kopin

Facsimile: (847) 562-9031

E-mail: notices@cranshirecapital.com

Intracoastal Capital LLC

245 Palm Trail

Delray Beach, Florida 33483

Attn: Mitchell Kopin/Keith Goodman

Facsimile: (847) 562-9031

E-mail: mkopin@cranshirecapital.com and kgoodman@cranshirecapital.com

Anson Investments Master Fund, Ltd.

111 Peter Street, Suite 904

Toronto, Ontario M5V 2H1

Canada

Attn: Amin Nathoo, Moez Kassam

E-mail: anathoo@ansonfunds.com

Capital Ventures International

c/o Heights Capital Management

101 California Street, Suite 3250

San Francisco, CA 94111,

Attn: Sam Winer, Martin Kobinger

E-mail: sam.winer@sig.com

Heng Hong Ltd.

Rams Office Complex

Stoney Grove, Box 822

Charlestown, Nevis

St. Kitts & Nevis, West Indies

Attn: Dan McAllister, Daniel MacMullin

E-mail: dmacmullin@ifgnevis.com

Punit Dhillon

11220 Corte Belleza

San Diego, CA 92130

Attn: Punit Dhillon

E-mail: pdhillon@idhillon.com

Ocean Creation Investments Ltd.

65 Chulia Street, #41-02

Singapore 049513

Attn: Norman Winata

E-mail: norman@lucrumcapital.com.sg

Ong Kim Kiat

4 Sunrise Drive

Singapore 806507

Attn: Ong Kim Kiat

E-mail: drkkong@yahoo.com

Karmdeep and Harpreet Bains

925 Woodridge Court

Yuba City, CA 95993

Attn: Karmdeep Bains

E-mail: karmbains1@comcast.net

Greenberg Traurig, LLP

77 W. Wacker Drive, Suite 3100

Chicago, Illinois 60601

Facsimile: (312) 456-8435

EXHIBIT A

Warrant Amendments

AMENDMENT TO SERIES A WARRANTS AND SERIES C WARRANTS TO PURCHASE COMMON STOCK

This Amendment to Series A Warrants and Series C Warrants to Purchase Common Stock (the “Amendment”) by and between Arch Therapeutics, Inc., a Nevada corporation (the “Company”), and Cranshire Capital Master Fund, Ltd. (“Cranshire”) is made as of June 22, 2015 (the “Effective Date”).

RECITALS

WHEREAS, on January 30, 2014, the Company entered into that certain Securities Purchase Agreement (the “Securities Purchase Agreement”) with the investors named on the Schedule of Buyers attached thereto (collectively, the “Holders”) pursuant to which, among other things, the Company issued to the Holders shares of Common Stock (as defined in the Series A Warrants and Series C Warrants (each as defined in the Securities Purchase Agreement)) and the Series A Warrants and Series C Warrants;

WHEREAS, Section 2(b) of the Series A Warrants and Series C Warrants provides that upon certain issuances or sales or deemed issuances or sales of shares of Common Stock for a consideration per share (the “New Issuance Price”) at less than a price equal to the Exercise Price in effect immediately prior to such issuance or sale or deemed issuance or sale (a “Dilutive Issuance”), the Exercise Price in effect immediately prior to such Dilutive Issuance shall be reduced to an amount equal to the New Issuance Price (the “Anti-Dilution Adjustment Provision”);

WHEREAS, the Expiration Date of the Series C Warrants is 5:00 p.m., New York time, on July 2, 2015;

WHEREAS, Section 9 of the Series A Warrants and Series C Warrants provides that the provisions of the Series A Warrants and Series C Warrants may be amended only with the written consent of the Company and the Significant Buyers (as defined in the Securities Purchase Agreement);

WHEREAS, Cranshire is the only Significant Buyer;

WHEREAS, the Company and Cranshire, in its capacity as the only Significant Buyer, now wish to amend (i) the Series A Warrants and Series C Warrants to terminate the Anti-Dilution Adjustment Provision; and (ii) the Series C Warrants to extend the Expiration Date, in each case on the terms and conditions set forth herein (collectively, the “Warrant Amendments”); and

WHEREAS, in consideration for Cranshire entering into the Warrant Amendments, the Company has agreed to offer to issue to the Holders or their successors and assigns, as applicable, the Inducement Shares (as defined below), on the terms and conditions set forth herein.

NOW, THEREFORE, in consideration of the foregoing and for good and valuable consideration, the receipt and sufficiency which are hereby acknowledged, the Parties hereby agree as follows:

“(b)     [Reserved]”

“(b)    [Reserved]”

“(h)    “Expiration Date” means 5:00 p.m., New York time, on July 2, 2016.”

[signature page follows]

IN WITNESS WHEREOF, Cranshire and the Company have caused their respective signature page to this Amendment to be duly executed as of the date first written above.

Exhibit A

Form of Investor Certificate

Arch Therapeutics, Inc.

235 Walnut St., Suite 6

Framingham, MA 01702

Ladies and Gentlemen:

Reference is made to that certain Amendment to Series A Warrants and Series C Warrants to Purchase Common Stock (the “Amendment”) by and between Arch Therapeutics, Inc., a Nevada corporation (the “Company”), and Cranshire Capital Master Fund, Ltd. dated as of June 22, 2015 (the “Amendment”). Capitalized terms not otherwise defined in this Investor Certificate shall have the meaning given to them in the Amendment.

As a condition to receiving the undersigned’s allocation of the Inducement Shares in accordance with the Amendment and as set forth on Exhibit B thereto (the “Allocation Schedule”), the undersigned hereby represents and warrants to the Company as follows:

THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR APPLICABLE STATE SECURITIES LAWS. THE SECURITIES MAY NOT BE OFFERED FOR SALE, SOLD, TRANSFERRED OR ASSIGNED (I) IN THE ABSENCE OF (A) AN EFFECTIVE REGISTRATION STATEMENT FOR THE SECURITIES UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR (B) AN OPINION OF COUNSEL TO THE HOLDER (IF REQUESTED BY THE COMPANY), IN A FORM REASONABLY ACCEPTABLE TO THE COMPANY, THAT REGISTRATION IS NOT REQUIRED UNDER SAID ACT OR (II) UNLESS SOLD OR ELIGIBLE TO BE SOLD PURSUANT TO RULE 144 OR RULE 144A UNDER SAID ACT. NOTWITHSTANDING THE FOREGOING, THE SECURITIES MAY BE PLEDGED IN CONNECTION WITH A BONA FIDE MARGIN ACCOUNT OR OTHER LOAN OR FINANCING ARRANGEMENT SECURED BY THE SECURITIES.

[signature page follows]

IN WITNESS WHEREOF, the undersigned has caused this Investor Certificate to be duly executed as of the date set forth below.

Exhibit B

Allocation of Inducement Shares

* Assignee of Equitec Specialists, LLC

** Assignee of 0903746 B.C. Ltd