Osteoma is a slow-growing, benign and uncommon neoplasm located primarily in the region of the maxillofacial skeleton. An extraskeletal soft-tissue osteoma is exceedingly rare. Here, we report a case of soft-tissue osteoma occurring in the pterygomandibular space in a 66-year-old woman. The patient complained of a hard mass superior to the left posterior teeth. Clinical examination did not reveal any extraoral swelling, facial asymmetry or difficulty in mouth opening, and the regional lymph nodes were non-palpable. CT images revealed well-circumscribed dense, radiopaque masses located between the left side of the maxilla and the lateral plate of the left pterygoid process and the left ramus of the mandible. Intraoperatively, the masses were completely surrounded by soft tissues with no attachment to the bone. Histological examination indicated the diagnosis of cancellous osteoma.