Quadricuspid aortic valve is a rare congenital condition that occurs not only as an isolated anomaly, but also with other cardiac defects. We describe a 10-year-old boy whose aortic stenosis was diagnosed during infancy. Transthoracic echocardiography revealed dilation of the left ventricle, valvular and subvalvular aortic stenosis, bicuspid aortic valve, aortic regurgitation, and mitral valve prolapse. The results of cardiac catheterization and aortography showed severe aortic regurgitation, an aortic valve gradient of 76 mmHg, a bicuspid aortic valve, a subaortic membrane, and an ascending aortic aneurysm. The patient underwent elective valve replacement with a mechanical prosthesis, and during surgery, the valve was noted to be quadricuspid. The patient was diagnosed as having a quadricuspid aortic valve associated with aortic regurgitation, severe aortic stenosis, and an ascending aortic aneurysm.