Iliopsoas abscess (IPA) is rare, especially in the neonatal period. The major presenting symptoms of IPA are leg or groin swelling, limitation of leg motion, and pain. The etiologies of IPA in many cases remain unknown, and the etiologic agent in many cases is Staphylococcus aureus. Ultrasonography and computed tomography are useful in diagnosing this disease. Antibiotics therapy and appropriate drainage are effective, and the prognosis is good. The authors present a neonatal case of IPA and discuss clinical symptoms, etiologic agents, methods of diagnosis and therapy, and prognosis.