The Wildlife Conservation Society, with the Cambodian Fisheries Administration and Wildlife Reserves Singapore, announced the Monday release at the Sre Ambel River.
Officials, conservationists, and local residents attended the release ceremony, a WCS release said.
The female turtle, captured in the Sre Ambel River by local fishermen in April 2011, is thought to be one of only about 200 adult terrapins remaining in the wilds of Cambodia, Malaysia, and Indonesia, and the population in the Sre Ambel River is estimated at fewer than 10 nesting females.
On its capture it was voluntarily turned it over to the WCS Cambodia turtle team instead of being sold on the black market and sent to food markets in China.
The released turtle, weighing about 75 pounds, has been fitted with a satellite transmitter to allow conservationists to monitor its movements, the first such monitoring of this species, researchers said.