Unroofed coronary sinus is a rare congenital cardiac anomaly in which a communication occurs between the coronary sinus and the left atrium due to the partial or complete absence of roof of the coronary sinus. Its diagnosis is important to prevent pulmonary hypertension and brain abscess or cerebral emboli. We report a case of a symptomatic 35-year-old man in which no atrial septal defects were visualized by transesophageal echocardiography but MRI permitted the diagnosis of this rare cardiac anomaly and assessment of shunt severity.