Presents a case of primary gastric undifferentiated sarcoma, where the patient, an elderly male with a history of type 2 diabetes and recurrent melena, was admitted with severe anemia. Gastroscopy revealed a 4 cm×4 cm irregular mass in the gastric antrum, and biopsy indicated ulceration, necrosis, and mild atypical hyperplasia. CT scans showed a 7 cm×3 cm×2.6 cm tumor involving the full thickness of the gastric wall. The patient underwent partial gastrectomy, and postoperative pathology confirmed high-grade undifferentiated sarcoma without lymph node metastasis. Recovery was smooth, and follow-up is ongoing. This case underscores the importance of thorough diagnostics and highlights surgery as the primary treatment, with close postoperative monitoring to prevent recurrence.