We present a rare case of thymoma that developed entirely in the middle mediastinum. A 60-year-old woman was referred to our hospital with a mediastinal mass. The mass was located in the visceral compartment of Shields' mediastinal classification. Intraoperative pathologic evaluation revealed a thymoma, whereupon total thymectomy was performed. The postoperative course was uncomplicated and the patient received no postoperative adjuvant radiotherapy.