We describe the case of a 74-year-old woman who had a left atrial mass that was accidentally found by preoperative transthoracic echocardiography. We performed transesophageal echocardiography (TEE) for evaluation of the left atrial mass. TEE revealed that the tumor, 24×18 mm in diameter, was attached to the left atrial wall near the superior vena cava and the aorta. The left atrial cavity was filled with spontaneous echo contrast. We suspected that the diagnosis of the left atrial mass was thrombus because the patient was in chronic atrial fibrillation; however, we could not rule out the possibility of a tumor such as a myxoma because the mass was partially attached to the atrial septum. Warfarin was started at a dose of 3 mg a day, with the international normalized ratio of prothrombin time being adjusted to a range of 1.6 to 2.0. We ultimately diagnosed the mass as a left atrial thrombus because the mass completely disappeared with continuation of anticoagulation therapy for 5 months without any embolic events. TEE is useful for the differential diagnosis of left atrial masses.