The Sanje mangabey is a highly endangered Old World monkey of the white-eyelid mangabey group from the Eastern Arc Mountains in Tanzania. They are about 50–65 centimetres (20–26 in) in length, excluding the tail, and their body colour is greyish. Fruit makes up about 70% of their diet. They live in valley forests and on mountain slopes, but are mostly ground-dwelling, which makes them susceptible to hunting and poaching. Their habitat is being degraded, and the International Union for Conservation of Nature has assessed their conservation status as being "endangered".

The white-eyelid mangabeys are African Old World monkeys belonging to the genus Cercocebus. They are characterized by their bare upper eyelids, which are lighter than their facial skin colouring, and the uniformly coloured hairs of the fur. The other two genera of mangabeys, Lophocebus and Rungwecebus, were once thought to be very closely related to Cercocebus, so much so that all the species were placed in one genus, but Lophocebus and Rungwecebus species are now understood to be more closely related to the baboons in genus Papio, while the Cercocebus species are more closely related to the mandrill.