Gastrointestinal stromal tumors arise from mesenchymal cells of gastrointestinal tract and are seen rarely. In fewer than 5% of cases they arise from outside of gastrointestinal tract, they originate primarily from mesentery, omentum and peritoneum. These are called extra- gastrointestinal tumors and they share the histological features with their gastrointestinal counterparts. A 57-year-old male patient was admitted to our hospital with complaint of abdominal pain. In his physical examination, an abdominal mass was detected in his left lower quadrant. Radiological examinations revealed an abdominal mass measuring 13×9 cm in diameter, adjacent to the bladder. Laparotomy demonstrated a lobulating mass of about 13×10 cm in diameter, originating from mesenterium of the sigmoid colon. En-bloc resection of the tumor with sigmoidectomy and end-to-end anastomosis was performed. Histopathology of the specimen showed that the tumor demonstrating high cellularity composed of spindle-shaped tumor cells. There was no continuity of tumor within the sigmoid colon and the margins of specimen were tumor-negative. Immunohistochemically, the tumor was positive for c-kit (CD 117). The tumor was diagnosed as a gastrointestinal stromal tumor originating from mesenterium of the sigmoid colon. A gastrointestinal stromal tumor of the sigmoid mesenterium is a rare entity. Surgical resection is the treatment of choice for these tumors, which usually have a poor prognosis.