Forest rangers say the primates are black-shanked douc langurs, a species classified as endangered by the International Union for Conservation of Nature and protected by Vietnamese law.
About 20 individuals living in two troops have been found on Mount Chua Chan at a height of about 300 meters (980 feet). A third troop was discovered further up the mountain at a height of about 600 meters, according to Ton Ha Quoc Dung, deputy head of Xuan Loc District and Long Khanh Town's inter-district Forest Protection Unit.
All three troops had pregnant females as well as infants, Vietnam News Agency quoted Dung as saying.
The local forest protection unit is currently implementing measures to protect the langurs and instructing locals not to hunt them.
Farmers working around Mount Chua Chan reported spotting the langurs searching for food on the mountain, and said they did not seem scared when they encountered humans.
Black-shanked, red-shanked and grey-shanked douc langurs are native to Vietnam, Cambodia and Laos, according to the World Wildlife Fund (WWF). Their populations have dropped by 50 to 80% over the last three decades, the WWF said on its website.
In Vietnam, the main threats to the douc langurs are habitat loss and hunting, and the animals are used for medicine, meat or sold as pets.