It unseated the previous record-holder, a 646lb (293kg) Mekong giant catfish caught in Thailand in 2005.
There is no official record-keeping or database of the world’s biggest freshwater fish.
The Mekong is rich in biodiversity but overfishing, dams and pollution threaten its fragile ecosystem.
It flows from the Tibetan Plateau through China, Myanmar, Thailand, Laos, Cambodia and Vietnam.
“In 20 years of researching giant fish in rivers and lakes on six continents, this is the largest freshwater fish that we’ve encountered or that’s been documented anywhere worldwide,” said Zeb Hogan, a biologist who leads Wonders of the Mekong, a USAID-funded conservation project.
“Finding and documenting this fish is remarkable, and a rare positive sign of hope, even more so because it occurred in the Mekong, a river that’s currently facing many challenges,” added Dr Hogan, who is also a professor at the University of Nevada, Reno.
The conservation project works with the Cambodian Fisheries Administration to set up a network of fishermen who alert the researchers if they catch giant or endangered fish.
On the night of 13 June, a local fisherman on Koh Preah island called to tell researchers that he had caught a “very big” stingray – it turned out to be 3.98m long and 2.2m wide.
After being fitted with an acoustic tag to track its future movement, the stingray was released back into the river.
It disappeared into the muddy waters of the Mekong around dusk when the moon was already up in the sky, Dr Hogan said.
In the local Khmer language, the fish is called “Boramy”, which means full moon.
“The stingray find is evidence that the natural world can still yield new and extraordinary discoveries, and that many of the largest aquatic creatures remain woefully understudied,” Dr Hogan said.
The giant freshwater stingray is an endangered species. This is the second giant stingray examined by the team since May – the earlier one weighed 181kg.
“When record fish are found, it means the aquatic environment is still relatively healthy. This is in contrast to what we’ve seen in places like the Yantgze River, where scientists reported the extinction of the Chinese paddlefish,” Dr Hogan said.
“The Mekong’s deep pools sustain life far beyond these impressive giants. Spawning in this critical habitat produces billions of fish every year which ensure the food security and livelihoods for millions of people in Cambodia and Vietnam.”