We present a rare case of middle mediastinal thymoma with myasthenia gravis. A 51-year-old man presented with right ptosis and muscle weakness, and received a diagnosis of generalized myasthenia gravis. Computed tomography of the chest showed a 20-mm nodule in the middle mediastinum, suggesting a possible ectopic thymoma. He underwent video-assisted thoracoscopic extended thymectomy and resection of the tumor. Histologic examination revealed an ectopic thymoma and ectopic thymic tissue around the tumor. One year after the operation, his condition remains well controlled solely with tacrolimus. Careful preoperative radiologic examination concerning possible ectopic thymoma outside the dissection area of the extended thymectomy is recommended.