A woman in her 50s visited our hospital in February 2015 with a complaint of dull abdominal pain in the right lower quadrant. She had a medical history of appendectomy for appendicitis in her 20s. Computed tomography(CT)revealed a tumor 90 mm in diameter near the ileocecum. Elective surgery was planned under the suspicion of gastrointestinal tumor, malignant lymphoma, or ileal cancer. She was emergently hospitalized 1 day earlier than scheduled because of high fever and severe abdominal pain. CT revealed that the tumor had increased to 120 mm in diameter without free air. Her white blood cell count was not elevated, and her symptoms improved readily with medical treatment. Thus, we performed the operation as scheduled. A tumor with a dark red recess on the surface had invaded the transverse colon intraoperatively, and a small amount of purulent ascites was present at the pouch of Douglas. We performed ileocecal resection with partial transverse colectomy. Histopathological examination led to the diagnosis of desmoid tumor in the mesentery of the terminal ileum. The surgical margins were negative for tumor cells. The tumor surface around the recess showed peritonitis, and the ascites showed no bacteria or tumor cells. The patient had been doing well without recurrence after discharge. Some cases of desmoid tumor with peritonitis have been reported, but most were caused by tumor penetration into the intestinal tract. We report herein a rare case of intra-abdominal desmoid tumor with abacterial peritonitis.