Cover Page - $ / shares	9 Months Ended
	Mar. 31, 2023	May 08, 2023
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Entity Registrant Name	CFSB Bancorp, Inc.
Document Period End Date	Mar. 31, 2023
Title of 12(b) Security	Stock, Par Value $0.01 Common Per Share
Document Fiscal Year Focus	2023
Security Exchange Name	NASDAQ
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--06-30
Entity Central Index Key	0001879103
Securities Act File Number	001-41220
Entity Tax Identification Number	87-4396534
Entity Incorporation, State or Country Code	X1
Entity Address, Address Line One	15 Beach Street
Entity Address, City or Town	Quincy
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02170
City Area Code	617
Local Phone Number	471-0750
Entity Current Reporting Status	Yes
Trading Symbol	CFSB
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		6,632,642
Entity Listing, Par Value Per Share	$ 0.01