A gossypiboma is a mass of cotton sponge left in the body postoperatively. Here, we report a case of gossypiboma with bleeding through a fistula to the colon, which became clinically evident 24 years after gynecological surgery, and resembled a bleeding diverticulum at colonoscopy. A 67-year-old woman presented with anemia and hematochezia. She had undergone a hysterectomy for myoma uteri 24 years earlier. Colonoscopy showed a deep depressed lesion mimicking a diverticulum with bleeding in the transverse colon. A contrast-enhanced computed tomography was interpreted as revealing a 6-cm thick-walled tumor, containing an air bubble, and a fistula between the mass and the transverse colon. The patient underwent laparotomy, with the preoperative expectation that the mass was a penetrating submucosal tumor. Pathological findings revealed denatured cotton tissues surrounded by reactive tissues to the foreign body. Despite its rarity, gossypiboma should be considered in patients with an intra-abdominal mass who have a history of laparotomy. Gossypiboma can cause fistula to the colon and bleeding. Imaging studies and the clinical course may mimic a malignant tumor.