| Cover - USD ($) | 9 Months Ended | ||
| Sep. 30, 2021 | Dec. 16, 2021 | Mar. 31, 2021 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Amendment Flag | false | ||
| Document Annual Report | true | ||
| Document Transition Report | false | ||
| Document Period End Date | Sep. 30, 2021 | ||
| Document Fiscal Period Focus | FY | ||
| Document Fiscal Year Focus | 2021 | ||
| Current Fiscal Year End Date | --09-30 | ||
| Entity File Number | 000-54986 | ||
| Entity Registrant Name | ARCH THERAPEUTICS, INC. | ||
| Entity Central Index Key | 0001537561 | ||
| Entity Tax Identification Number | 46-0524102 | ||
| Entity Incorporation, State or Country Code | NV | ||
| Entity Address, Address Line One | 235 Walnut Street | ||
| Entity Address, Address Line Two | Suite 6 | ||
| Entity Address, City or Town | Framingham | ||
| Entity Address, State or Province | MA | ||
| Entity Address, Postal Zip Code | 01702 | ||
| City Area Code | 617 | ||
| Local Phone Number | 431-2313 | ||
| Title of 12(g) Security | Common Stock, par value $0.001 per share | ||
| Entity Well-known Seasoned Issuer | No | ||
| Entity Voluntary Filers | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Interactive Data Current | Yes | ||
| Entity Filer Category | Non-accelerated Filer | ||
| Entity Small Business | true | ||
| Entity Emerging Growth Company | false | ||
| Entity Shell Company | false | ||
| Entity Public Float | $ 32,000,000 | ||
| Entity Common Stock, Shares Outstanding | 237,169,770 |
