The Mandrill (Mandrillus sphinx) is a primate of the Cercopithecidae (Old-world monkeys) family, closely related to the baboons and even more closely to the Drill. Both the Mandrill and the Drill were once classified as baboons in genus Papio, but recent research has determined that they should be separated into their own genus, Mandrillus. The Mandrill is the world's largest monkey species. The word mandrill means "man-ape".

The Mandrill is recognized by its olive-colored fur and colorful face and rump amongst males, a coloration that grows stronger with sexual maturity; females have duller colors. This coloration becomes more pronounced as the monkey becomes excited. The coloration on the rump is thought to enhance visibility in the thick vegetation of the rainforest and aids in group movement.

Males can weigh up to 60 lb (30 kg), females about half as much. They can grow to be about 1 m long (39 in) and can survive up to 25 years in captivity. Females reach sexual maturity at about 3.5 years.

The Mandrill is found in the tropical rainforests of West Africa (Southern Nigeria, South Cameroon, Gabon, and Congo). It is a social creature and may be found in groups ranging from 5 to 50 individuals, led by an older dominant male. Six to seven of these groups may come together during the dry season to form a troop of over 200 individuals.

The Mandrill is an omnivore and acquires its food by foraging (mainly plants, insects and smaller animals) from the ground as it is terrestrial, although it may climb trees occasionally to sleep. Its main natural predators are leopards and cheetahs. Because of over-hunting by humans, the Mandrill has become endangered. The situation is exacerbated by deforestation causing Mandrill habitats to disappear.

Although the Mandrill does not normally hunt larger prey, on some occasions males have been observed to hunt and consume duiker (a small antelope). Despite this it is well adapted to fighting and is not to be provoked.