Odontogenic sinusitis is a unilateral infection of the maxillary sinus caused by a dental origin. An ectopic tooth in the maxillary sinus is a rare situation that can cause sinusitis as well as dentigerous cysts. Diagnosis is based on a combination of clinical symptoms and radiographic findings. We report the case of a 40-year-old woman who presented with headache and hemifacial pain revealing a chronic sinusitis caused by a dentigerous cyst associated with ectopic third molar that was managed medically and surgically.