Gastrointestinal stromal tumors (GISTs) are a mesenchymal tumor of the digestive tract and they have various clinical characteristics. We report here on the largest extragastric pedunculated GIST of the stomach that has been seen in Korea. The patient was a 67-year-old man with a giant abdominal mass occupying the whole abdomen, and both leg showed swelling for the previous several months. On computed tomography (CT) and magnetic resonance imaging (MRI), this appeared as a septated cystic tumor with a solid component. Laparotomy revealed a giant extragastric tumor arising from the lesser curvature of the stomach that measured 47x34x23 cm and it weighed about 40 kg. Surgical treatment was performed to remove both the giant mass and the gastric wall where the tumor was attached to a 3-cm pedicle. On immunohistochemistry, the tumor was positive for myeloid stem cell antigen (CD34) and c-kit (CD117). The final diagnosis was a pedunculated extragastric type GIST arising from the stomach. The postoperative course was uneventful and the swelling in both legs resolved.