A case of undifferentiated pleomorphic sarcoma in the mediastinum is presented. A 74-year-old man with no complaint was referred to our department for treatment of mediastinal mass pointed out by chest X-ray. Chest computed tomography (CT) revealed 3 tumors located in the left mediastinum. The largest one was adjacent to the esophagus with 10 cm in diameter. The other one was located beneath the left inferior pulmonary vein, and the last adjacent to the pulmonary artery trunk. Complete extirpation of the tumors was unsuccessful because of the invasion to the pulmonary artery trunk. A diagnosis of undifferentiated pleomorphic sarcoma was made with negative immunohistochemical staining for markers except for vimentin. Tumor cells were found to be scattered in the partially resected thymus. We assume this is a rare presentation of mediastinal dissemination of the tumor.