| Cover Document | Sep. 28, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Sep. 28, 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 |
| Title of 12(b) Security | Common Stock, no par value |
| Trading Symbol | UBFO |
| Security Exchange Name | NASDAQ |
