The mandrill (Mandrillus sphinx) is a primate of the Old World monkey (Cercopithecidae) family, closely related to the baboons and even more closely to the drill. It is found in southern Cameroon, Gabon, Equatorial Guinea, and Congo. Mandrills mostly live in tropical rainforests and forest-savanna mosaics. They live in groups called "hordes". Mandrills have an omnivorous diet consisting mostly of fruits and insects. Their mating season takes place from June to October.

Both the mandrill and the drill were once classified as baboons in genus Papio, but recent research has determined they should be separated into their own genus, Mandrillus. The mandrill is the world's largest species of monkey. Charles Darwin wrote, "no other member in the whole class of mammals is coloured in so extraordinary a manner as the adult male mandrills". The mandrill is classified as vulnerable by IUCN.