The primate was discovered in the Democratic Republic of Congo where it is known locally as a "lesula".
It is the second new species of African monkey discovered in the last 28 years and has 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," researchers described the species.
Scientists first saw the monkey when they encountered a juvenile female, kept in a cage by a primary school director in the town of Opala, BBC Nature reported.
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.
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"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," Dr John Hart of the Lukuru Foundation, who led the project, said.
"We did not expect to find a new species, especially in a group as well known as the African guenons," Hart said.
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.
"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.
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.
The discovery was published in the journal Public Library of Science.