Document and Entity Information - USD ($)	12 Months Ended
	Jun. 30, 2021	Sep. 10, 2021	Dec. 31, 2020
Cover page [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Current Fiscal Year End Date	--06-30
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Entity Registrant Name	GREENE COUNTY BANCORP INC
Entity Central Index Key	0001070524
Entity Address, State or Province	NY
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 77,649,000
Entity Common Stock, Shares Outstanding		8,513,414