Cor triatriatum sinister is a rare congenital cardiac anomaly, occurring in approximately 1 in 1,000 congenital cases. Although typically diagnosed in infancy because of significant haemodynamic consequences, cases in adulthood are uncommon and may present with nonspecific symptoms.

A 31-year-old male presented with progressive dyspnoea and haemoptysis. Transthoracic echocardiography revealed a fibrous membrane dividing the left atrium, with a 5-mm fenestration permitting communication between an accessory chamber and the main left atrial cavity; four pulmonary veins drained into the accessory chamber. In addition, an atrial septal defect and pulmonary hypertension were identified. The patient underwent surgical correction, which included resection of the membrane, closure of the atrial septal defect using a Dacron patch, and DeVega tricuspid annuloplasty. Intraoperative transesophageal echocardiography confirmed absence of residual shunt and tricuspid regurgitation with improved pulmonary artery pressures.

This case underscores the necessity for early recognition and timely surgical intervention in adult cor triatriatum sinister cases to prevent complications such as right ventricular failure and severe pulmonary hypertension.