We report a case of idiopathic omental bleeding in a 27-year-old man who was brought to our hospital after the sudden development of intermittent abdominal pain, nausea, and fainting. Computed tomography showed intra-abdominal fluid and emergency laparotomy revealed a hemorrhagic mass in the omental bursa, which was excised. The patient was successfully treated and a diagnosis of idiopathic omental bleeding was made.