A 40-year-old woman complained of chest pain. The diagnosis was chest aortic aneurysm and a stent graph was inserted. Initially, the chest pain disappeared, but it later recurred. Chest contrast-enhanced computed tomography (CT) revealed a mass in the mediastinum and a nodule in the lungs. Endoscopic ultrasound-guided fine needle aspiration (EUS-FNA) was performed. The tumor was composed of spindle-shaped cells or pleomorphic cells with a high N/C ratio. The tumor cells stained for vimentin, but not for CK, AE1/AE3, α-SMA, or S-100. At this point, the diagnosis was undifferentiated sarcoma. The patient received chemotherapy with weekly paclitaxel and radiation therapies. These therapies had no effect. Doxorubicin alone and ifosfamide plus doxorubicin showed tumor shrinkage. However, the tumor formed a fistula from the esophagus to the thoracic aorta, and consequently the patient died of hematemesis. The tumor was diagnosed as an undifferentiated pleomorphic sarcoma (UPS) at autopsy. The UPS was called a malignant fibrous histiocytoma (MFH) conventionally and was classified as an unclassifiable/undifferentiated sarcoma in accordance with the fourth edition of WHO classifications published in 2013. UPS/MFH occur most frequently in the retroperitoneal space and extremities. This report will focus on this rare case of mediastinal UPS.