A 27-year-old female patient presented to the emergency room with abdominal pain and vomiting. The clinical findings including the computed tomography (CT) scan, suggested internal herniation. The patient had no history of previous abdominal operation or peritonitis. At the surgery, a part of the distal jejunum and proximal ileum 90 cm in length was herniated through a 4 cm-sized round mesenteric defect of the mesentery. Spontaneous transmesenteric hernia is a rare cause of internal herniation, but the possibility should be kept in mind even when the patient has no history of abdominal operation or peritonitis.