Dentigerous cysts are the most common developmental odontogenic cyst of the jaws. They are more frequent in males and although most often affected teeth are maxillary canines and mandibular third molars, they may also be related with supernumerary or an ectopically erupted tooth. Our aim was to report the management of ectopic third molar tooth related with a dentigerous cyst in maxillary sinus. 16 years old girl referred to our clinic with a complaint of painful swelling over her right cheek since past six months. All permanent teeth were present except the right upper third molar. External examination showed that the face was gently asymmetrical; levels of eye globes were abnormal. Radiographic examination revealed a cystic lesion related with ectopic third molar which located in the roof of the left maxillary sinus. Under local anesthesia, hard drain was inserted into the hole and the cystic pressure was decreased. After the six month of followup, the operation was performed under general anesthesia. The patient's symptoms were resolved completely after surgery and remained symptom-free for over a postoperative follow-up period of 3 year. To avoid unwanted effects of a dentigerous cyst, unerupted tooth should be observed with radiographic imaging. Some untreated dentigerous cysts may grow large and have a potential to develop into an odontogenic tumor.