An isolated quadricuspid aortic valve is an extremely rare congenital anomaly and there have been few surgical case reports published. A 47-year-old man with untreated diabetes mellitus was admitted to our institution because of fever and dyspnea. Transesophageal echocardiography showed severe aortic valve regurgitation and a quadricuspid valve with vegetations. Blood culture revealed Streptococcus agalactiae. Despite administration of antibiotics and treatment of his heart failure, the infection and heart failure were not controlled. Therefore, we performed aortic valve replacement in the presence of active infective endocarditis. The aortic valve had 2 equal-sized larger cusps and 2 equal-sized smaller cusps. There were vegetations on each cusp and an annular abscess was detected. The resection site of the abscess was reinforced with an autologous pericardial patch, and the aortic valve was replaced using a 21-mm SJM valve. His postoperative course was uneventful and he was discharged after recovery.