Dentigerous cysts are odontogenic lesions associated with the crown of permanent teeth. These lesions are nonmalignant. Dentigerous cysts, which surround impacted teeth, can cause teeth to become ectopic. In the maxilla, impacted teeth associated with dentigerous cysts are often displaced towards the maxillary sinus cavities. These lesions can be successfully treated with Caldwell-Luc surgery and/or endoscopic sinus surgery. We report a case of a dentigerous cyst in the maxillary sinus associated with an ectopic third molar and its treatment.