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.
The first contact scientists had with the monkey was when they encountered a juvenile female, kept in a cage by a primary school director in the town of Opala.
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." After genetic analysis identified the species as a member of the guenon group of Old World monkeys, scientists named it Cercopithecus lomamiensis after the nearby Lomami River.
"The challenge for conservation now in Congo is to intervene before losses become definitive," said Dr Hart. "Species with small ranges like the lesula can move from vulnerable to seriously endangered over the course of just a few years."
Biologists suggested that the previously unexplored forest could be home to more unidentified species. "This discovery may be only the first from this remarkable but poorly known forest, located in the central DRC [DR Congo]," said anthropologist Andrew Burrell from New York University who was also involved in the study.
"Recent surveys have shown that the forest also harbours okapi, bonobos and elephants, as well as 10 other primate species or subspecies." Dr Hart added that the region is now in the final stages of being declared a protected area: the Lomami National Park.
"The discovery of the lesula has extended our knowledge of the evolution and ecology of African monkeys, and in particular has confirmed the importance of a previously little-known region for primate diversity," he told BBC Nature.
