Cover	9 Months Ended
	Sep. 30, 2020 shares
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Entity File Number	000-32897
Entity Registrant Name	UNITED SECURITY BANCSHARES
Entity Incorporation, State or Country Code	CA
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
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding	16,977,239
Entity Central Index Key	0001137547
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false
Document Period End Date	Sep. 30, 2020