We report a case of gastrointestinal stromal tumor (GIST) arising from the rectal mesentery. GIST of the large intestine is a rare tumor that accounts for only 0.1% of all colorectal cancers. The patient presented to our hospital with constipation and abdominal distension. Computed tomography (CT) revealed a huge mass in the pelvic cavity, and laparotomy disclosed diffuse peritoneal dissemination from the primary tumor. Radiochemotherapy was commenced, but the patient became too ill to complete it and died of the disease 2 months after surgery. A large high-grade tumor with diffuse dissemination was recognized as an indicator of poor survival in this patient.