We report an extremely rare case of undifferentiated pleomorphic sarcoma of the stomach. A 74-year-old woman was admitted for abdominal discomfort. A 13-cm gastric submucosal tumor and multiple liver and bone metastases were detected by computed tomography. The tumor had not been detected 8 months earlier. We performed EUS-FNAB for a suspected mesenchymal tumor, but the immunohistochemical test (c-kit, CD34, desmin, SMA, S-100) findings were negative. The tumor was confirmed as wild type for c-kit and PDGFRα. It grew rapidly and the patient died 2 months after admission. Pathological analysis of the EUS-FNAB specimens and autopsy revealed an undifferentiated pleomorphic sarcoma.