We report herein a rare case of gastrointestinal stromal tumor (GIST) type, arising from the greater omentum. A 65-year-old man who had a large abdominal tumor was referred to our hospital. Ultrasonography (US) and computed tomography (CT) scans showed a mass occupying almost the entire abdomen anterior to the bowel loops. Abdominal angiography showed that the main feeding artery of the tumor was the right gastroepiploic artery. The preoperative diagnosis was suspected gastric leiomyosarcoma. Laparotomy revealed a large mass arising from the greater omentum, and the tumor seemed to be completely excised. Histopathological and immunohistochemical studies indicated the tumor had the same characteristics as GIST. Twelve months after the operation, the tumor recurred in the peritoneal cavity at the site of the stomach, and was associated with multiple liver metastases. The patient died of hypovolemic shock. Necropsy revealed that rupture of one of the metastatic liver tumors had resulted in a massive intraperitoneal hemorrhage.