An 82-year-old man visited our hospital for abdominal discomfort and constipation lasting 3 days. He did not have a history of trauma or bleeding tendency. His laboratory studies showed slight anemia and inflammatory reactions, but other data including coagulation profiles were all normal. Computed tomography (CT) demonstrated a 6-cm high-density mass in the left upper quadrant and slight ascites. Contrast-enhanced CT revealed that the lesion was not enhanced. Colonoscopy could not detect the lesion. Laparotomy was performed because malignancy could not be ruled out. The mass was located in the transverse colon, and histologic examination showed intramural hematoma. Neither arteriovenous malformation nor a foreign body such as a fish bone was identified, indicating that the mass was considered to be idiopathic intramural colonic hematoma. We present this rare case and also a review of the literature.