A 45-year-old female was admitted to our hospital due to severe aortic stenosis. Transthoracic and transesophageal echocardiography indicated the aortic valve was bicuspid, but we confirmed that the aortic valve was unicuspid by intraoperative finding. There was 1 open commissure between the left coronary cusp and the noncoronary cusp position. Aortic valve replacement was performed with a mechanical valve, and the postoperative course was uneventful. We report a very rare case of unicuspid aortic valve replacement.