A case of acute torsion of an accessory spleen is reported. In a 15-year-old girl was admitted to hospital with acute abdominal pain. Ultrasonography showed a round, hypoechoic, solid mass. Computed tomography demonstrated a low-density mass with peripheral enhancement after intravenous contrast medium. These findings suggest the presence of a capsule. At laparotomy, the patient was found to have torsion and infarction of an accessory spleen. Sonographic and computed tomographic findings are discussed.