Torsion of an accessory spleen is an extremely rare condition. We describe an unusual case of acute abdomen caused by torsion of an accessory spleen in a 12-year-old boy. The patient underwent a laparotomy with splenectomy; the course was favorable. We discuss the clinical findings and values of preoperative instrumental diagnosis. The literature is also reviewed. This is the 11th case reported in the English literature. Torsion of an accessory spleen should be considered in the differential diagnosis of acute abdomen or subacute abdominal pain.