A 32-year-old woman complained of acute lower abdominal pain. Computed tomography showed a complex multilocular cystic mass at the right adnexal region. Magnetic resonance imaging demonstrated the origin of the mass to be the small bowel mesentery. Chemical-shift images detected septal fat of the cystic mass and suggested a small amount of fat within the locules of the cyst. A cystic tumor of the mesentery such as cystic lymphangioma, hemangioma, cystic mesothelioma, and dermoid was included in the differential diagnoses. The diagnosis of a hemorrhagic mesenteric cystic lymphangioma was confirmed at surgery and pathologic analysis. Cystic lymphangioma should be included in the differential diagnosis of acute abdominal pain. The detection of septal fat may be helpful in the diagnosis of cystic lymphangioma when it shows unusual radiological appearances.