The blue monkey or diademed monkey (Cercopithecus mitis) is a species of Old World monkey native to Central and East Africa, ranging from the upper Congo River basin east to the East African Rift and south to northern Angola and Zambia and populations further south down to South Africa. The taxonomy of this species has been disputed and Sykes' monkey, the silver monkey and the golden monkey are often regarded as subspecies.

Several subspecies are recognised:

At times, some of these have been regarded as full species, and additional subspecies have been considered valid, while others are not recognized by all authorities.

Like other mammals with the name "blue" (blue tick hound, blue wildebeest), the blue monkey often has a slaty-grey cast that can look blue to observers. It is mainly olive or grey apart from the face (which is dark with a pale or yellowish patch on the forehead – the "diadem" from which the species derives its common name), the blackish cap, feet, and front legs, and the mantle, which is brown, olive, or grey depending on the subspecies. Typical sizes range from 50 to 65 cm (20 to 26 in) in length, (not including the tail, which is almost as long as the rest of the animal), with females weighing a little over 4 kg (8.8 lb) and males up to 8 kg (18 lb).

The blue monkey is found in evergreen forests and montane bamboo forests, and lives largely in the forest canopy, coming to the ground infrequently. It is very dependent on humid, shady areas with plenty of water. It eats mainly fruit and leaves, but will take some slower-moving invertebrates. It prefers to live in tall trees, which provide both food and shelter, and is, therefore, like almost all guenons, suffering from the loss of its natural habitat. Where pine plantations replace natural forest, the monkey may be treated as a threat by foresters, since it sometimes strips bark from exotic trees in a search for food or moisture. It is also hunted for bushmeat and as payback for crop-raiding.

Blue monkeys primarily eat fruits, including figs. They also eat insects, leaves, twigs, and flowers. "They are primarily frugivores, with 50% of their diet consisting of fruit, with leaves or insects being the main source of protein, with the rest of the diet being made up of seeds, flowers and fungi. [....] They eat a variety of plants but concentrate on a few species, which means their population density is generally dependent on plant species richness and diversity." Rarely, they eat vertebrates, such as lizards, birds, and small mammals including bushbabies.

Cercopithecus mitis joins with the C. ascanius (red-tailed monkey) for extra protection. Its interactions with red-tailed monkeys include interspecies grooming. Their social system is mainly female because the males leave once they are mature. Males have little to no interaction with the young. C. mitis is very territorial, so young males must leave. They may challenge the dominant male of another family. If they defeat the dominant male, they take over leadership of that family, and this offers a place to live, socialization, and food supplies.

The blue monkeys live in female-philopatric social systems where females stay in their natal groups, while males disperse once they reach adulthood. As a result, blue monkey groups usually consist of one male with several females and infants, giving rise to matrilinear societies. Occasionally, solitary males are observed, which are probably transient, having left their natal group in search of a new group.