We report a case of preoperative spontaneous regression of thymoma in a 66-year-old woman who presented chest pain. Chest computed tomography revealed a well-defined tumor in the anterior mediastinum. The long axis of the tumor decreased from 25 to 18 mm during 1 month preoperatively. Video-assisted thoracoscopic thymothymectomy was performed for definitive diagnosis and treatment. Histopathologically, the tumor mainly comprised necrotic components with partially viable cells, and was diagnosed as a thymoma. The occlusion of the feeding artery by organized thrombus was found by pathology and it was considered to be the cause of coagulative necrosis.