Abstract Osteoid osteoma is rarely found in the acetabular fossa. We treated a case of osteoid osteoma that had developed into a mass lesion and protruded from the acetabular fossa with synovitis. In this case, a well-defined bony tumor located in the acetabular fossa was detected by plain radiographs and computed tomography scans. Gadolinium-enhanced T1-weighted magnetic resonance (MR) images obtained with fat saturation indicated distinctive features in which the central area of the lesion and hyperemic synovium were enhanced. A surgical specimen contained the nidus, and osteoid osteoma was diagnosed. Retrospectively, this MR sequence demonstrated the characteristic nidus and synovitis, and it may thus be valuable in cases where diagnosis of intra-articular osteoid osteoma is difficult.