Osteomas attached to the meninges unrelated to bone are extremely rare and their etiology has not been discussed previously in the English literature. Here, we report the case of a 54-year-old male patient with a right frontal subdural osteoma. The patient presented with a ~5-month history of intermittent dizziness. Non-contrasted computerized tomography demonstrated a densely calcified mass attached to the inner surface of the right frontal skull. Magnetic resonance imaging (MRI) revealed this mass could be enhanced by contrast MRI. Intraoperatively, the hard mass was noted to be attached to the inner layer of the dura mater, and not associated with the bone. Histopathological examination revealed lamellated bony trabeculae lined by osteoblasts and the intertrabecular marrow spaces occupied by adipose tissue, which contributed to the MRI enhancement. It was speculated that subdural osteomas arose from ectopic osteoblasts derived from the embryological neural crest cells. The context of intertrabecular bone marrow contributed to the enhancement on MRI. Simple excision is the treatment of choice for symptomatic lesions.