Extraskeletal osteosarcoma (ESOS) arising from the mediastinum is a rare malignant tumor and associated with a poor prognosis. We present the case of a 73-year-old man with a hoarseness. Imaging studies revealed a large calcified tumor of the median mediastinum. Surgery was performed, but complete resection was impossible and about two-thirds of the tumor was excised. The tumor was pathologically diagnosed as ESOS. Proton beam therapy has been performed on the remaining lesion, and the patient is alive without tumor regrowth for 29 months. This is the first case of mediastinum ESOS successfully treated with surgery and postoperative proton therapy.