Torsion of an accessory spleen is an uncommon cause of abdominal pain. Only a few cases have been reported in the literature. Most cases occur in children, and in most cases the diagnosis is made at surgery. We report a case of torsion of an accessory spleen in an adult female who presented with acute left-flank pain. The diagnosis was made on contrast-enhanced computed tomography (CT) and was confirmed at surgery. Without treatment, torsion of an accessory spleen can lead to hemorrhagic shock, peritonitis, and bowel obstruction. It is useful for the radiologist to make this diagnosis on imaging so that biopsy can be avoided and surgery performed to avoid complications.