A rare case of primary cardiac undifferentiated pleomorphic sarcoma (UPS) is reported. A 77-yearold female was admitted to the authors' hospital with complaints of palpitation and dyspnea on effort. Echocardiography revealed a mobile tumor arising from the atrial septum of the left atrium. The tumor obstructed the mitral valve and the patient experienced acute heart failure. Emergent open-heart surgery was performed to resect the tumor. Although she was discharged from hospital on postoperative day 14 in a satisfactory condition, local recurrence in the left atrium was observed 16 months after surgery. Repeated tumor resection was performed, and histological examination confirmed UPS. The patient did not agree to undergo chemotherapy or radiation therapy and died of local recurrence 27 months after the first surgery.