Unroofed coronary sinus syndrome is an extremely rare congenital heart anomaly. A 21-year-old male patient with unroofed coronary sinus syndrome diagnosed by coronary angiography is presented in this case report. Coronary sinus blood was shunting into the left atrium via a small defect in the middle part of the coronary sinus. Surgical treatment was not considered as the amount of shunting was small, and the patient was put on follow-up.