We herein report a case of complex odontoma arising in the maxillary sinus. A 61-year-old woman visited our hospital with a chief complaint of pain in her right buccal region, and post-nasal discharge. Computed tomography showed a huge high density mass on the posteroinferior side of the right maxillary sinus and soft tissue density on the upper side of the maxillary sinus, which was thought to reflect maxillary sinusitis. The mass was surgically removed under endoscopic assistance using the Caldwell-Luc procedure and endoscopic surgery was performed to repair the maxillary sinus. The pathological diagnosis was a complex odontoma associated with chronic sinusitis. The patient is doing well at approximately 1 year after surgery.