Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2021	Aug. 03, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Entity Registrant Name	GTJ REIT, INC.
Entity Central Index Key	0001368757
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Document Quarterly Report	true
Document Transition Report	false
Entity Common Stock, Shares Outstanding		13,773,853
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Tax Identification Number	20-5188065
Entity File Number	333-136110
Entity Incorporation, State or Country Code	MD
Entity Address, Address Line One	60 Hempstead Avenue
Entity Address, City or Town	West Hempstead
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	11552
City Area Code	516
Local Phone Number	693-5500