Miller's Grizzled Langur is known for its black face, white collar and hooded eyes.
A team of scientists set up cameras in the Wehea Forest on the eastern tip of Borneo Island in June, hoping to snap images of clouded leopards, orangutans and other wildlife known to be in the region.
The pictures that came back caught them by surprise: groups of monkeys none had ever seen. The only existing images were museum sketches.
The monkey once roamed the northeastern part of Borneo, as well as the islands of Sumatra and Java and the Thai-Malay peninsula. But concerns were voiced several years ago that they may be extinct.
Forests where the monkeys once lived had been destroyed by fires, human encroachment and conversion of land for agriculture and mining and an extensive field survey in 2005 turned up empty.
The next step will be returning to the 90,000 acre (38,000 hectare) forest to try to find out how many grizzly langurs there are, according to the team of local and international scientists, who published their findings in the American Journal of Primatology on Friday.
The Associated Press contributed to this report.