Heterotopic ossification (HO) refers to the mature lamellar bone structure that develops locally in soft-tissue sites. Heterotopic ossification is known to occur mainly in the traumatic area or the muscle and joint capsule after surgery such as total hip arthroplasty or total knee arthroplasty. The pathophysiological cause of HO is not precisely known, but it seems to be related to some genetic predisposition. Craniomaxillofacial HO is rare and, in several reported cases, is primarily related to previous surgeries involving the masticatory muscles. The authors report on a patient who underwent endoscopic removal of a bone mass in the forehead. He completely denied any history of forehead trauma or surgery. There were no scars, trauma, or surgical marks found on the patient’s forehead. Preoperative magnetic resonance imaging showed that the mass was granuloma, but histological examination after surgical removal confirmed it as HO.