Unroofed coronary sinus (UCS) represents a rare subtype of atrial septal defect (ASD), an adult congenital heart disease characterized by communication between the systemic and pulmonary circulations at the atrial level. This case report presents the unique occurrence of a large, unrepaired ASD secundum and an incidentally identified UCS type II in a 25-year-old female during a cardiac murmur assessment. The diagnosis of ASD secundum was initially made using transthoracic echocardiography (TTE) and was later confirmed with a transesophageal echocardiogram. The identification of the UCS was achieved through the utilization of cardiac computed tomography angiography (CCTA). Ultimately, the patient underwent a successful reroofing procedure using a bovine pericardial patch.