A 67-year-old man with a history of aortic mechanical valve replacement exhibited an abnormal mass on the sewing ring of the prosthesis on echocardiography. Despite receiving strong anticoagulation, he developed acute cerebral infarction due to the formation of emboli resulting from the thrombus and underwent urgent re-aortic valve replacement. Based on the microscopic findings of the resected mass, he was finally diagnosed as having nonbacterial thrombotic endocarditis (NBTE) of the mechanical prosthetic valve, which was thought to be associated with colorectal cancer. We herein report the first known case of an antemortem diagnosis of NBTE on a mechanical heart valve.