The Delacour's langur, black and white with a full face of whiskers, is indigenous to Vietnam, but their numbers have dwindled in recent years because of poaching and mining activity in the country's northern forests.
A team of scientists from Fauna and Flora International spotted a group of about 40 of the primates, mostly juveniles and infants, bringing their total population to less than 250.
"It's great news for this particular species because had we not found this new population, they were in grave danger of being wiped out within a decade," spokeswoman for FFI in Vietnam, Akofa Wallace, told AFP yesterday.
FFI did not say where scientists spotted the langurs, whose habitat is threatened by mining activity in the area, including charcoal production.
More From This Section
They are also targeted by poachers who hunt them for meat, with their bones used for traditional medicine and their pelts for decoration.
The primate was discovered in northern Vietnam in the 1930s by French scientist Jean Theodore Delacour, and are only found in Vietnam.
FFI country director Benjamin Rawson said urgent interventions were needed to protect the species, which numbered about 300 in the early 1990s.
The rare animals are protected under Vietnam's conservation laws, but critics say the legislation is not effectively enforced and poaching of rare or endangered species continues unchecked.
Vietnam is home to some of world's most endangered species, including the mountainous antelope Saola, the Red River giant soft-shell turtle and the Tonkin snub-nosed monkey.
Wild animals are under constant threat in the country, with their body parts in high demand for both food and traditional medicine.