Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2024	May 03, 2024
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2024
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Entity Registrant Name	FIRST US BANCSHARES INC
Entity Central Index Key	0000717806
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		5,788,672
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Document Quarterly Report	true
Document Transition Report	false
Entity File Number	0-14549
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	63-0843362
Entity Address, Address Line One	3291 U.S. Highway 280
Entity Address, City or Town	Birmingham
Entity Address, State or Province	AL
Entity Address, Postal Zip Code	35243
City Area Code	205
Local Phone Number	582-1200
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	FUSB
Security Exchange Name	NASDAQ