Document and Entity Information - shares	3 Months Ended
	Sep. 30, 2020	Nov. 10, 2020
Cover page.
Entity Registrant Name	GREENE COUNTY BANCORP INC
Entity Central Index Key	0001070524
Current Fiscal Year End Date	--06-30
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		8,513,414
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Document Quarterly Report	true
Document Transition Report	false
Entity Address, State or Province	NY