Cor triatriatum is a rare congenital cardiac anomaly in which the accessory chamber is separated from the left atrium by an anomalous septum. We report a rare case of cor triatriatum with severe mitral regurgitation and atrial fibrillation in an adult. The patient was a 65-year old woman who developed congestive heart failure 3 years previously, and received medical follow-up with mitral regurgitation, atrial fibrillation and cor triatriatum since then. She developed congestive heart failure again and was referred to our hospital for operation for progressed mitral regurgitation, tricuspid regurgitation and atrial fibrillation. Mitral valve plasty (Physio ring II 28 mm, cleft closure, edge to edge repair for PMC), tricuspid annuloplasty (Physio tricuspid ring 28 mm), resection of the anomalous septum and maze procedure was performed. All of the pulmonary veins were connected to the accessory chamber. There was only one hole on the anomalous septum, and the hole was large, about 3.0 cm in diameter. The patient regained sinus rhythm without mitral and tricuspid regurgitation after the operation. Even though the duration of atrial fibrillation was long and left atrium diameter was large, complete excision of the anomalous septum and maze procedure were effective for the patient diagnosed cor triatriatum.