Obturator internus muscle (OIM) abscess is an uncommon condition often mistaken for bacterial infection of the hip joint. If the OIM abscess is accompanied by a septic hip, it becomes more difficult to make a diagnosis. Surgical drainage of OIM abscess is difficult because it is located at a deep part of the pelvic cavity. Therefore, intravenous antibiotic therapy or image-guided aspiration were used as a treatment method instead of surgical drainage. When drainage of the abscess was inadequate, prolonged antibiotic treatment was necessary. Here, we report a 12-year boy with abscess in the OIM and ipsilateral pyogenic arthritis of the hip which was accompanied by acute osteomyelitis of the triradiate cartilage of the acetabulum. The patient was successfully treated by surgical drainage of the abscess using the Stoppa approach and arthroscopic irrigation of the hip joint. Staphylococcus aureuswas the causative organism which was sensitive to oxacillin.