Cor triatriatum sinister is a rare congenital cardiac anomaly in which the left atrium is divided into two chambers by a fibromuscular septum. Although the disease is generally diagnosed in early childhood, some patients remain asymptomatic until adulthood. A 19-year-old female patient presented with dyspnea that increased in severity for the past year. Her functional capacity was NYHA class II. She had no other complaints in history. Cardiac auscultation revealed a grade 2/6 apical diastolic murmur. Findings of electrocardiography and chest radiography were normal. Transthoracic echocardiography showed a thin membrane dividing the left atrium, but no transmembrane flow was visualized. The flow could be visualized by transesophageal echocardiography, but the area of membrane fenestration could not be determined. The diagnosis was made as cor triatriatum sinister and the patient underwent cardiac catheterization, which revealed a pressure gradient of 10 mmHg between the pulmonary capillary wedge pressure and left ventricular end-diastolic pressure. The patient was submitted to surgical correction.