Osteoma is an uncommon tumor found in various animal species and is particularly rare or underreported in ferrets. A 2-year-old neutered male ferret (Mustela putorius furo) presented with an asymmetrical firm mass affecting the right dorsolateral skull. Radiographic images and computed tomography scans revealed a well-marginated mineralized mass involving the dorsal right calvarium that was growing concentrically into the cranium. A surgical biopsy sample was obtained and yielded a cytological diagnosis of sarcoma with reactive osteoblasts and osteoclasts and a histopathological diagnosis of osteoma. It was suspected that only the outer bony rim of the mass was submitted for histopathological evaluation, thereby adversely affecting the diagnostic quality of the sample. Based on interpretation of the combined imaging, cytological, and histopathological findings, a multilobular tumor of bone was suspected. Because of reduction in quality of life, the animal was euthanized 4 months after presentation, and postmortem examination yielded a definitive diagnosis of osteoma. This is the first report of an osteoma with cranial vault invasion in a ferret and is one of very few reports of this tumor diagnosed in this species. Lack of neurological signs in the patient, marked and extensive growth of the mass over several months, and the similar clinical appearance to other bony tumors leading to diagnostic ambiguity contributed to the distinctiveness of this case.