The animal was discovered during a recent forest patrol in the Kim Hy Nature Reserve in Thai Nguyen Province, according to the reserve's management board.
Rangers said the primate had black fur, distinctive white markings running from its ears down its cheeks and a long tail, characteristics consistent with a white-cheeked langur.
Local residents said they had heard unusual calls, believed to have come from the animal, in the area at around 5-6 am since early August.
The reserve has increased patrols and assigned staff to monitor the animal, determine whether others are present and track its movements to support conservation measures.
Local authorities and forest rangers have also been asked to strengthen public awareness of wildlife protection.
Residents have been advised to keep a safe distance if they encounter the langur or other wild animals and not to approach, chase, feed or disturb them.
They have also been asked to report sightings to the reserve or concerned agencies so the animals can be monitored and protected.
