We report a rare case of a large thrombus in the ascending aorta with acute arterial occlusive disease. A 61-year-old man was transferred to our hospital with sudden pain and cyanosis. Contrast-enhanced computed tomography detected left ulnar arterial occlusion and a large mass in the ascending aorta, so we performed surgery to remove the large thrombus under cardiopulmonary bypass. Histologically, the mass was a fibrin thrombus. In addition, thickened endothelial lining and slight atheromatous degeneration was detected in the resected aortic wall. The patient was discharged from the hospital on postoperative day 22.