We encountered a 75-year-old man who complained of exertional dyspnea. An echocardiographic examination showed aortic regurgitation and a tumor in the left ventricular outflow tract. Under complete extracorporeal circulation, we surgically made an incision of the ascending aorta with a slight thickening of the aortic valve and an enlarged annulus. After excising the aortic valve, an examination of the subvalvular region revealed mitral valve-like tissue extending from the annular region of the right coronary cusp to the ventricular septum, while the chordae tendinae was attached to the septum. This issue was excised, and the aortic valve was replaced with a 27-mm SJM® valve. The postoperative course was uneventful, and the patient was discharged in good condition on postoperative day 30. An accessory mitral valve is extremely rare. Since this indication for surgical treatment is associated with congenital heart disease or a left ventricular outflow tract obstruction, most patients are young. Our patient had no associated cardiac anomalies and no pressure gradient attributable to a left ventricular outflow tract obstruction. This accessory mitral valve was discovered during aortic valve replacement surgery. To our knowledge, our patient is the oldest reported with an accessory mitral valve to have undergone a surgical resection.