Cor triatriatum is an uncommon congenital malformation in which the fibromuscular abnormal septum divides the atrium into two compartments. Generally it is used to mean cor triatriatum sinister in which the left atrium is divided into two lumens. In typical cor triatriatum, an abnormal fibromuscular septum divides a proximal chamber that receives the pulmonary vein from a distal chamber that communicates with the left atrial appendage and the mitral valve. Several hypothesis were proposed concerning the mechanism of the abnormal septation of the atrium, but no single hypothesis could explain all the cases convincingly. We report the successful surgical treatment of a 6-month-old male patient with the rare variant of the cor triatriatum with pulmonary hypertension. The left pulmonary vein and superior right pulmonary vein drained into an accessory chamber which opened to the right atrium through a sinus venousus type atrial septum defect. The left inferior pulmonary vein drained into the true lumen which opened to right atrium through the patent foramen ovale.