Filed Pursuant to Rule 424(b)(3)

Registration No. 333-194745

PROSPECTUS SUPPLEMENT NO. 12 DATED SEPTEMBER 30, 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. 12 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 additions and changes:

This Prospectus Supplement No. 12 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. 12 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. 12 is September 30, 2015

INDEX TO FILINGS

ANNEX A

AMENDMENT TO SELLING SECURITYHOLDER INFORMATION

This Prospectus Supplement No. 12 is being filed in connection with several changes to the selling securityholder information as set forth in the Prospectus dated May 22, 2015 (as supplemented to date, the “Prospectus”) including, but not limited to, (i) the transfer by Capital Ventures International of its Series A Warrants, Convertible Note and its shares of our common stock to CVI Investments, Inc., an affiliate; (ii) sales of securities registered under the registration statement of which the Prospectus is a part by several of the selling securityholders; (iii) the issuance of shares of our common stock to the selling securityholders or their affiliates in connection with the June 22, 2015 amendment to our Series A Warrants and Series C Warrants; and (iv) the issuance of shares of our common stock and Series D Warrants to certain of the selling securityholders in the private placement that we concluded on July 2, 2015. Consequently, the selling securityholders table appearing under the heading “SELLING SECURITYHOLDERS” in the Prospectus, is hereby amended and supplemented by replacing that table and the accompanying footnotes in its entirety with the following:

Cranshire Capital Advisors, LLC (“CCA”) is the investment manager of Cranshire Capital Master Fund, Ltd. (“Cranshire Master Fund”) and consequently has voting control and investment discretion over the securities held by Cranshire Master Fund described below. Mr. Kopin, the president, the sole member and the sole member of the Board of Managers of CCA, has voting control over CCA. As a result, each of Mr. Kopin and CCA may be deemed to have beneficial ownership (as determined under Section 13(d) of the Exchange Act) of the securities held by Cranshire Master Fund that are described below.

Mr. Kopin and Mr. Asher may be deemed to have beneficial ownership (as determined under Section 13(d) of the Exchange Act) of 4,361,718 shares of common stock, which consists of (i) 1,590,909 shares of common stock issued to Intracoastal in connection with the 2015 Private Placement Financing, none of which are being registered in the registration statement of which this prospectus forms a part; (ii) 40,000 Inducement Shares issued to Intracoastal, none of which are being registered in the registration statement of which this prospectus forms a part; (iii) 50,000 shares of common stock that became issuable under the Series A Warrant assigned to Intracoastal in May 2015 by Equitec as a result of the anti-dilution provisions in the Series A Warrants that were triggered by the Notes Offering, none of which are being registered in the registration statement of which this prospectus forms a part; (iv) 1,590,909 shares of common stock issuable upon exercise of Series D Warrants issued to Intracoastal in connection with the 2015 Private Placement Financing, none of which are being registered in the registration statement of which this prospectus forms a part; and (v) assuming that the remaining principal on the Convertible Note originally issued to Equitec and assigned to Intracoastal in May 2015 (the “Intracoastal Convertible Debenture”), and the accrued interest thereunder is converted on March 13, 2016 into shares of our common stock at the conversion rate in effect as of the date of this prospectus, 1,089,900 shares of common stock issuable upon the conversion of the Intracoastal Convertible Debenture, none of which are being registered in the registration statement of which this prospectus forms a part.

Mr. Asher, who is a manager of Intracoastal, is also a control person of a broker-dealer. As a result of such common control, Intracoastal may be deemed to be an affiliate of a broker-dealer. Intracoastal acquired the shares of common stock being registered hereunder in the ordinary course of business, and at the time of the acquisition of the shares of common stock and Series A and Series C Warrants described herein, Intracoastal did not have any arrangements or understandings with any person to distribute such securities.

CCA may be deemed to have beneficial ownership (as determined under Section 13(d) of the Exchange Act) of 360,000 shares of common stock, which consists of (i) 160,000 Inducement Shares issued to Cranshire Master Fund, none of which are being registered in the registration statement of which this prospectus forms a part; and (ii) 200,000 shares of common stock that became issuable under the Series A Warrant issued to Cranshire Master Fund in connection with the Private Placement Financing as a result of the anti-dilution provisions in the Series A Warrants that were triggered by the Notes Offering, none of which are being registered in the registration statement of which this prospectus forms a part.

As a result of the foregoing, Mr. Kopin may be deemed to have beneficial ownership (as determined under Section 13(d) of the Exchange Act) of 4,721,718 shares of common stock, which consists of (i) 1,590,909 shares of common stock issued to Intracoastal in connection with the 2015 Private Placement Financing, none of which are being registered in the registration statement of which this prospectus forms a part; (ii) 40,000 Inducement Shares issued to Intracoastal, none of which are being registered in the registration statement of which this prospectus forms a part; (iii) 160,000 Inducement Shares issued to Cranshire Master Fund, none of which are being registered in the registration statement of which this prospectus forms a part; (iv) 50,000 shares of common stock that became issuable under the Series A Warrant assigned to Intracoastal in May 2015 by Equitec as a result of the anti-dilution provisions in the Series A Warrants that were triggered by the Notes Offering, none of which are being registered in the registration statement of which this prospectus forms a part; (v) 200,000 shares of common stock that became issuable under the Series A Warrant issued to Cranshire Master Fund in connection with the Private Placement Financing as a result of the anti-dilution provisions in the Series A Warrants that were triggered by the Notes Offering, none of which are being registered in the registration statement of which this prospectus forms a part; (vi) 1,590,909 shares of common stock issuable upon exercise of Series D Warrants issued to Intracoastal in connection with the 2015 Private Placement Financing, none of which are being registered in the registration statement of which this prospectus forms a part; and (vii) assuming that the remaining principal on the Intracoastal Convertible Debenture and the accrued interest thereunder is converted on March 13, 2016 into shares of our common stock at the conversion rate in effect as of the date of this prospectus, 1,089,900 shares of common stock issuable upon the conversion of the Intracoastal Convertible Debenture, none of which are being registered in the registration statement of which this prospectus forms a part.

Anson may be deemed to have beneficial ownership (as determined under Section 13(d) of the Exchange Act) of 5,269,568 shares of common stock, which consists of (i) 100,000 Inducement Shares issued to Anson, none of which are being registered in the registration statement of which this prospectus forms a part; (ii) 1,600,000 shares of common stock issued to Anson in connection with the 2015 Private Placement Financing, none of which are being registered in the registration statement of which this prospectus forms a part; (iii) 1,600,000 shares of common stock issuable upon exercise of Series D Warrants issued to Anson in connection with the 2015 Private Placement Financing, none of which are being registered in the registration statement of which this prospectus forms a part; (iv) 1,000,000 shares of common stock that became issuable under the Series A Warrant issued to Anson in connection with the Private Placement Financing as a result of the anti-dilution provisions in the Series A Warrants that were triggered by the Notes Offering, none of which are being registered in the registration statement of which this prospectus forms a part; (v) 390,666 shares of common stock issued to Anson upon the partial conversion of the outstanding principal on Anson’s Convertible Note, none of which are being registered in the registration statement of which this prospectus forms a part; and (vi) assuming that the remaining principal on the Convertible Note and the accrued interest thereunder is converted on March 13, 2016 into shares of our common stock at the conversion rate in effect as of the date of this prospectus, 578,902 shares of common stock issuable upon the conversion of the Convertible Note, none of which are being registered in the registration statement of which this prospectus forms a part. The foregoing excludes 330,709 shares of our common stock issuable upon conversion of the Convertible Note held by Anson because the Convertible Note contains ownership limitations as described in footnote (2), under which the holder thereof does not have the right to convert the Convertible Note to the extent (but only to the extent) that such conversion would result in beneficial ownership by the holder thereof or any of its affiliates of more than 4.99% of our common stock (which may be increased to 9.99% at the holder’s discretion). Without such ownership limitations, Anson would be deemed to have beneficial ownership of 5,600,277 shares of our common stock.

CVI, Heights Capital Management, Inc. and Mr. Kobinger may be deemed to have beneficial ownership (as determined under Section 13(d) of the Exchange Act) of 4,398,088 shares of common stock, which consists of (i) 100,000 Inducement Shares issued to Capital Ventures International (“Capital Ventures”), an investor in the Private Placement Financing and an affiliate of CVI, and assigned to CVI in September 2015, none of which are being registered in the registration statement of which this prospectus forms a part; (ii) 2,000,000 shares of common stock issuable under the Series A Warrant assigned to CVI in September 2015 by Capital Ventures; (iii) an additional 1,000,000 shares of common stock that became issuable under the Series A Warrant assigned to CVI in September 2015 by Capital Ventures as a result of the anti-dilution provisions in the Series A Warrants that were triggered by the Notes Offering, none of which are being registered in the registration statement of which this prospectus forms a part; (iv) 156,533 shares of common stock issued to CVI upon the partial conversion of the outstanding principal on the Convertible Note originally issued to Capital Ventures and assigned to CVI in September 2015, none of which are being registered in the registration statement of which this prospectus forms a part; and (v) assuming that the remaining principal on the Convertible Note and the accrued interest thereunder is converted on March 13, 2016 into shares of our common stock at the conversion rate in effect as of the date of this prospectus, 1,141,555 shares of common stock issuable upon the conversion of the Convertible Note, none of which are being registered in the registration statement of which this prospectus forms a part.

CVI is affiliated with one or more broker-dealers, none of which are currently expected to participate in the offering and resale pursuant to the prospectus contained in this registration statement of the shares held of record by CVI. CVI acquired the shares being registered hereunder in the ordinary course of business, and at the time of the acquisition of the shares and Warrants described herein, CVI did not have any arrangements or understandings with any person to distribute such securities.

Mr. MacMullin and Heng Hong may be deemed to have beneficial ownership (as determined under Section 13(d) of the Exchange Act) of 5,269,568 shares of common stock, which consists of (i) 2,100,000 shares of common stock issued to Heng Hong in the Private Placement Financing; (ii) 105,000 Inducement Shares issued to Heng Hong, none of which are being registered in the registration statement of which this prospectus forms a part; (iii) 2,100,000 shares of common stock issuable under the Series A Warrant issued to Heng Hong in the Private Placement Financing; and (iv) 964,568 shares of common stock issuable under the Series C Warrant issued to Heng Hong in the Private Placement Financing. The foregoing excludes (1) 1,135,432 shares of common stock issuable under the Series C Warrant issued to Heng Hong in the Private Placement Financing because the Series C Warrants contains ownership limitations as described in footnote (2); and (2) an additional 1,050,000 shares of common stock that became issuable under the Series A Warrant issued to Heng Hong in connection with the Private Placement Financing as a result of the anti-dilution provisions in the Series A Warrants that were triggered by the Notes Offering, none of which are being registered in the registration statement of which this prospectus forms a part, because the Series A Warrants contains ownership limitations as described in footnote (2). Without such ownership limitations, Daniel MacMullin and Heng Hong would be deemed to have beneficial ownership of 7,455,000 shares of our common stock.

Mr. Winata and OCIL may be deemed to have beneficial ownership (as determined under Section 13(d) of the Exchange Act) of 710,000 shares of common stock, which consists of (i) 200,000 shares of common stock issued to OCIL in the Private Placement Financing; (ii) 10,000 Inducement Shares issued to OCIL, none of which are being registered in the registration statement of which this prospectus forms a part; (iii) 200,000 shares of common stock issuable under the Series A Warrant issued to OCIL in the Private Placement Financing; (iv) 200,000 shares of common stock issuable under the Series C Warrant issued to OCIL in the Private Placement Financing; and (v) 100,000 shares of common stock that became issuable under the Series A Warrant issued to OCIL in connection with the Private Placement Financing as a result of the anti-dilution provisions in the Series A Warrants that were triggered by the Notes Offering, none of which are being registered in the registration statement of which this prospectus forms a part

Except for the ownership of the common stock, Warrants issued in the Private Placement Financing, the Convertible Notes issued in the Notes Offering, the shares common stock and Series D Warrants issued to Cranshire Master Fund and Anson in the 2015 Private Placement Financing, and as otherwise described in the table above and this “Selling Securityholder” section, (a) we have not made, and are not required to make, any potential payments regarding the Private Placement Financing to any selling securityholder, any affiliate of a selling securityholder, or any person with whom any selling securityholder has a contractual relationship, and (b) none of the selling securityholders has, or has had within the past three years, any material relationship with us or any of our predecessors or affiliates. Additionally, none of the selling securityholders holds any of our securities, other than those issued in the Private Placement Financing or the 2015 Private Placement Financing, that have been registered under the Securities Act or that are entitled to registration rights thereunder. We have also been advised that none of the selling securityholders is a broker-dealer or an affiliate of a broker-dealer, other than Mr. Asher, who is a manager of Intracoastal and also a control person of a broker-dealer, and CVI Investments, Inc., each of which has informed us that it is an affiliate of a broker-dealer.

The holders of the Series A and Series C Warrants issued in the Private Placement Financing have ongoing rights to exercise those Series A and Series C Warrants. We have described the material terms of the Series A and Series C Warrants elsewhere in this prospectus. In addition, the participants in the Private Placement Financing have ongoing registration rights related to the securities issued therein pursuant to the terms of the Registration Rights Agreement, which are described in more detail elsewhere in this prospectus.

We have made no payments, in cash or equity, to any of the selling securityholders in connection with this offering, except that we have reimbursed, or have agreed to reimburse, Cranshire Master Fund, one of the selling securityholders, an aggregate cash amount of up to $35,000 for costs and expenses incurred by it or its affiliates in connection with the transactions contemplated by the Private Placement Financing and the registration of the securities being registered hereby. Additionally, we may be required to make certain payments to the investors in the Private Placement Financing under certain circumstances in the future pursuant to the terms of the Securities Purchase Agreement and the Registration Rights Agreement. These potential future payments include the following: (a) potential partial damages for failure to register and keep registered for the period specified in the Registration Rights Agreement the common stock issued in the Private Placement Financing or issuable upon exercise of the Warrants (in a cash amount equal to 1% of the price paid to us by each investor in the Private Placement Financing on the date of and on each 30-day anniversary of such failure until the cure thereof, with no quantitative cap to the aggregate amount of such payments (provided that no such payments will be owed, other than with respect to a suspension or delisting of our common stock from the OTCQB, with respect to any period during which an investor’s Registrable Securities may all be sold without restriction under Rule 144)); (b) amounts payable if we or our transfer agent fails to timely remove certain restrictive legends from certificates representing shares of common stock issued in the Private Placement Financing or issuable upon exercise of the Warrants; (c) the portion of the $35,000 expense reimbursement to Cranshire Master Fund that has not been paid as of the date of this prospectus; and (d) payments in respect of claims for which we provide indemnification in the Securities Purchase Agreement and the Registration Rights Agreement. Although we intend to comply with the requirements of the Securities Purchase Agreement and the Registration Rights Agreement and do not currently expect to make any such payments, it is possible that such payments may be required.

The Securities Purchase Agreement entered into in connection with the Private Placement Financing granted to the investors, until the six month anniversary of the first date on which all the Registrable Securities (as defined in the Securities Purchase Agreement) are covered by one or more effective registration statements, the right to participate in any financing by us through an issuance of any of our securities up to an amount equal to the pro rata portion of the investor’s subscription amount in the Private Placement Financing, on the same pricing and other terms and conditions as such subsequent financing, provided that the aggregate participation by all such investors collectively shall not exceed 50% of the subsequent financing amount. This right expired on January 2, 2015. The terms and conditions of such subsequent financing shall not include any provision that requires a participating investor to agree to any restrictions on its trading of any of the shares acquired in connection with the Private Placement Financing without such investor’s consent.