The animal is almost entirely black with white tufts on its ear and chin, according to the wildlife organisation Fauna & Flora International (FFI), whose primatologists were involved in the discovery.
The monkeys "display characteristics unlike any other snub-nosed species previously described", it said. Although the species is new to science, it is well known to locals who call it the "monkey with an upturned face".
On rainy days they often sit with their heads tucked between their knees to avoid getting water in their upturned noses. The monkeys are believed to spend the summer at higher altitudes in mixed temperate forests and descend closer to villages in winter when food in scarce.
Frank Momberg, FFI's Asia Pacific regional programme development coordinator, said interviews with local hunters suggested there were just 260-330 individuals, which would make it critically endangered.
The monkey inhabits an area in Kachin State that is geographically isolated from other species by the Mekong and the Salween rivers, which may explain why it has not been discovered earlier, FFI said.
Species of snub-nosed monkeys are found in parts of China and Vietnam and considered endangered. Conservationists say this is the first time species have been reported in Myanmar.