The primate was discovered in the Democratic Republic of Congo where it is known locally as a "lesula".
The species is separated from its nearest cousins by two rivers: the Congo and the Lomami.
Conservationists say the discovery highlights the need to protect the diverse wildlife of the Congo basin.
The discovery was published in the online journal Public Library of Science.
He referred to the animal as a "lesula", a common name among local hunters, and it was taken into care and monitored by scientists.
During investigations in the local area the team found further captive monkeys and six months later they finally observed the long black limbs of the species in the wild.
"When we started our inventories in the [Tshuapa-Lomami-Lualaba] landscape we knew it was essentially unexplored but we did not imagine how important the biological discoveries would be," said Dr John Hart of the Lukuru Foundation, who led the project.
"We did not expect to find a new species, especially in a group as well known as the African guenons."
In the paper, the formal description of the species detailed their distinctive facial features: "A mane of long grizzled blond hairs frames a protruding pale, naked face and muzzle, with a variably distinct cream-coloured vertical nose stripe."