The Horton Plains slender loris is a small, nocturnal animal which can grow up to 17 centimetres (six inches) long with big, bulging eyes.
Endemic to Sri Lanka, it was first discovered in 1937 but had only been seen four times since then.
Scientists last caught a glimpse of the primate in 2002, and believed the elusive animal had since died out.
But field researchers, working with the Zoological Society of London, managed to track down the mysterious creature in the forests of central Sri Lanka.
In a world first, they were able to take pictures of an adult male slender loris sitting on a tree branch.
The field team was able to capture one of the creatures and give it a physical examination, the first time that has ever been done, before releasing it back into the wild.
But experts warned that deforestation in Sri Lanka — largely blamed on the drive to create tea plantations in the region — was now the biggest threat to the loris.
Craig Turner, a conservation biologist at the ZSL, said their natural forest habitat had been divided up for farming and logging use, cutting off the “very cute” animals from their partners.
“The forest has now been fragmented into a series of small islands,” Turner told BBC radio.
“They can’t move to one another, they can’t mate, breed, so it has real implications for the future persistence of the species.”