Cor triatriatum sinister is a rare congenital anomaly characterized by the presence of a fibromuscular membrane dividing the left atrium into two chambers: one entering the four pulmonary veins, the other connecting to the mitral valve. The extent of the communication between the two chambers and the presence of associated lesions determine the severity of symptoms and the complications. We report the case of a 20-year-old man firstly diagnosed with obstructive cor triatriatum sinister and severe pulmonary hypertension.