We describe the case of a 22-year-old woman who presented with a slowly growing osseous lesion of the mastoid cortex. On computed tomography, the lesion was found to involve the mastoid cortex, with which it demonstrated similar attenuation. The indications for treatment in this case were the patient's sensation of a mass effect, the encroachment of the mass onto the external auditory meatus, and a cosmetic deformity. The tumor was removed in its entirety via a postauricular approach. Findings on histopathologic examination were consistent with a compact osteoma. Mastoid osteomas are rare, benign tumors. If their growth significantly occludes the meatus, they may cause cosmetic deformities, conductive hearing loss, and recurrent external ear infections. Several other osseous lesions of the temporal bone should be considered in the differential diagnosis. The etiology of mastoid osteomas is poorly understood. Surgical management can be undertaken with minimal postoperative morbidity.