A coronary fistula (CF) is a rare congenital cardiac anomaly in which there is a connection between the coronary artery and a cardiac chamber or a great vessel. In the paediatric population, a CF is usually asymptomatic. While the circumflex coronary artery (Cx) is the least common source of a CF, the right heart chambers are the most common location of drainage. Herein, we present a symptomatic 10-month-old boy with an atrial septal defect (ASD) in whom we incidentally detected a CF, which stemmed from the Cx and drained to the right atrium. Because the patient was symptomatic and his small size was not appropriate for percutaneous closure of the ASD, surgical closure of the ASD and CF was performed.