| Document and Entity Information | Dec. 08, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Dec. 8, 2021 |
| Entity File Number | 000-10843 |
| Entity Registrant Name | CSP Inc. |
| Entity Incorporation, State or Country Code | MA |
| Entity Tax Identification Number | 04-2441294 |
| Entity Address, Address Line One | 175 Cabot Street |
| Entity Address, Adress Line Two | Suite 210 |
| Entity Address, City or Town | Lowell |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 01854 |
| City Area Code | 978 |
| Local Phone Number | 954-5038 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | CSPI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000356037 |
| Amendment Flag | false |
