A 39-year-old man had cor triatriatum (Lucas-Schmidt type IA) with severe mitral regurgitation and atrial fibrillation. We performed resection of the anomalous septum between the accessory chamber and the left atrium, and conducted mitral valve repair and the maze procedure. The patient regained sinus rhythm and normal pulmonary venous drainage to the left ventricle without mitral regurgitation. Histological examination demonstrated fibrotic myocardial structures in the anomalous septum. The maze procedure and complete excision of the anomalous septum proved to be effective surgical treatment for atrial fibrillation with cor triatriatum.