An 11-kilogram female Cantor's giant turtle - known for its rubbery skin and jaws powerful enough to crush bone - was captured and released by researchers in March, United States-based Conservation International and World Wildlife Fund said in a statement.
Scientists from the two groups and Cambodian wildlife officials discovered the animal and a nesting ground during a survey in Cambodia's Mekong River region.
'It has the fastest strike of any animal I've ever seen, including cobras'
"We thought it might be almost gone, but found it in abundance in this one pristine stretch of the Mekong, making the area the world's most important site for saving this particular species," David Emmett, a CI wildlife biologist, said in the statement.
"This incredible discovery means that a unique turtle can be saved from disappearing from our planet," he said.
The turtle is currently classified as endangered by the World Conservation Union, and continues to face threats from habitat loss and hunting.
The species can grow up to 2 metres in length and weigh more than 50 kilograms. It was last spotted by scientists in the Cambodian wild in 2003.
It was also found in small numbers in Laos, but appears to have disappeared from Vietnam and Thailand.
The turtle has a rubbery skin with ribs fused together to form a protective layer over its internal organs. It protects itself from predators by spending 95 percent of its life hidden in sand or mud with only its eyes and nose showing.
It also has long claws and can extend its neck with lightning speed to bite with jaws powerful enough to crush bone, the statement said.
"It has the fastest strike of any animal I've ever seen, including cobras," Emmett said.
The statement only said the turtle was found in a former Khmer Rouge stronghold long closed to scientific exploration because of security concerns. The communist Khmer Rouge ruled the country from 1975-1979 and their reign of terror resulted in the deaths of about 1,7 million people.
The area is "a near pristine region of tall riverine forest, waterways and island archipelagos where further exciting biological discoveries will almost certainly be made," Mark Bezuijen of WWF's Living Mekong Programme, who led the survey team, said in the statement.
It was the first detailed conservation study of the area since security restrictions were eased in the late 1990s, WWF said.
Bezuijen said the find highlights the need to protect the area from poachers and others who could threaten the turtles' habitat.
International and Cambodian turtle conservation officials will hire local villagers to protect nesting beaches for the turtles and conduct patrols during the dry season to prevent illegal fishing of the species, which is prized as an expensive delicacy in neighbouring Vietnam.
The villagers will also receive financial incentives to offset the potential loss of revenue from illegal trade in the turtles, WWF said. - Sapa-AP