A 14-year-old boy was brought to our hospital with abdominal pain and nausea after suffering a blow to the abdomen. A mass was felt in the right hypogastrium, and the patient was hospitalized for possible hematoma resulting from the abdominal trauma. Initially, we treated him conservatively and observed his course, but on the 20th day after trauma, enhanced computed tomography showed that the area of strong enhancement in the tumor was unchanged. Superior mesenteric angiography showed findings indicative of a pseudoaneurysm caused by the trauma, and surgery was performed 26 days after the injury. Laparotomy revealed a tumor with a clear boundary, thought to originate in the mesoappendix, without any sign of pseudoaneurysm. Histopathological examination confirmed that the tumor was an omental-mesenteric myxoid hamartoma. The patient had an uneventful postoperative course and was discharged from hospital on the 12th day after surgery. More than 5 years have elapsed since the operation and no sign of recurrence or metastasis has been recognized.