| Cover Document | Oct. 20, 2021 | Jun. 22, 2021 |
| Cover [Abstract] | ||
| Document Type | 8-K | |
| Document Period End Date | Oct. 20, 2021 | |
| Entity Registrant Name | UNITED SECURITY BANCSHARES | |
| Entity Incorporation, State or Country Code | CA | |
| Entity File Number | 000-32987 | |
| Entity Tax Identification Number | 91-2112732 | |
| Entity Address, Address Line One | 2126 Inyo Street | |
| Entity Address, City or Town | Fresno | |
| Entity Address, State or Province | CA | |
| Entity Address, Postal Zip Code | 93721 | |
| City Area Code | 559 | |
| Local Phone Number | 248-4943 | |
| Written Communications | false | |
| Soliciting Material | false | |
| Pre-commencement Tender Offer | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Entity Emerging Growth Company | false | |
| Entity Central Index Key | 0001137547 | |
| Amendment Flag | false | |
| Trading Symbol | UBFO | |
| Security Exchange Name | NASDAQ | |
| Title of 12(b) Security | Common Stock, no par value |
