A "substantial" population of 455 critically endangered northern white-cheeked crested gibbons were found living at high altitudes and far from human settlements on the border with Laos, Conservation International (CI) said.
Researchers, who had previously found sparse groups in other areas, used the animals` "loud, elaborate and prolonged" calls to locate the creatures in Pu Mat National Park in Nghe An province, northern Vietnam.
The community represents two thirds of the total number in Vietnam and the "only confirmed viable population" of the variety worldwide.
"This is an extraordinarily significant find, and underscores the immense importance of protected areas in providing the last refuges for the region`s decimated wildlife," said CI president Dr Russell Mittermeier.
Gibbons, which are threatened across the world, are considered the "most romantic" of primates as they mate for life and serenade their partners with song.
Habitat loss and hunting for the pet trade and the "assumed medicinal value of primate body parts" are among the major threats to the creatures in Vietnam, the CI statement said.
White-cheeked gibbon numbers are thought to have declined by as much as 80 percent in the last 45 years, according to the International Union for Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species.
Mittermeier, who also works with the IUCN, described the species as "on the brink of extinction".
They are believed to be "functionally extinct" in China and while there could be significant numbers in Laos, CI said a lack of research means the situation in the country is unclear.
But CI said plans to build a road through the Pu Mat area to increase patrols on the Vietnam-Laos border pose a "serious threat" to the future of the rare primates.
"The major issue will be the hunting of these gibbons that were previously protected by the harsh terrain; so gun control will be vital," said primatologist Luu Tuong Bach, a consultant to CI who led field surveys for the research.
"Without direct protection in Pu Mat National Park, it is likely that Vietnam will lose this species in the near future."