A 75-year-old man with moderate aortic stenosis and regurgitation admitted due to heart failure underwent uneventful aortic valve replacement with a Carpentier-Edwards pericardial bioprosthesis valve. A quadricuspid aortic valve discovered incidentally during surgery consisted of 4 of different sizes and a supernumerary cusp between the right and noncoronary cusps. No coronary abnormality was involved. Resected cusps showed fibrotic thickening with calcification and no sign of previous inflammatory disease. Although quadricuspid aortic valve is a very rare anomaly, its potential for severe valve failure in adulthood should not be neglected.