Introduction : The dentigerous cyst is a well-known congenital malformation usually found in the temporal region. This is the first case report of a large maxillary sinus cyst with ectopic teeth in an adult horse. Methods: A 14-year-old warmblood gelding was presented for evaluation of chronic right nasal discharge. After imaging exploration, surgery was performed on the standing horse with a conchofrontal sinus trephination and a large maxillary bone (MB) sinutomy. Results : The examinations revealed a sinusal mass in the right maxillary sinus covered by sinusal mucosa and around the infraorbital canal. The mass contained three ectopic tooth structures. The dental table was complete and normal. Both approaches (right frontal trephination, and maxillary sinutomy) showed adhesion between the dental structures, the infraorbital canal and the MB. The two simultaneous approaches allowed extraction with preservation of all sinus and nerve structures. Conclusions: The loss of integrity in the MB and the attachment to the infraorbital canal suggests that there could have been subsequent damage to the infraorbital canal, seen at the time of the examination and the surgery, leading to neurological symptoms in the future. The extraction of these completed structures in the standing horse has not been described and the use of both approaches allowed good visualization and preservation of the integrity of the sinus structures whilst allowing complete removal of the mass.