Christened Boramy - meaning "full moon" in the Khmer language - because of her bulbous shape, the four-metre long female was released back into the river after being electronically tagged to allow scientists to monitor her movement and behaviour.
"This is very exciting news because it was the world's largest fish," biologist Zeb Hogan, ex-host of the Monster Fish show on the National Geographic Channel and now part of a conservation project on the river, said.
"It is also exciting news because it means that this stretch of the Mekong is still healthy.... It is a sign of hope that these huge fish still live (here)."
Freshwater fish are defined as those that spend their entire lives in freshwater, as opposed to giant marine species such as bluefin tuna and marlin, or fish that migrate between fresh and saltwater like the huge beluga sturgeon.
"Yeah, when you see a fish this size, especially in freshwater, it is hard to comprehend so I think all of our team was stunned,"Â Hogan said a separate interview.
Boramy, netted last week off Koh Preah, an island along the northern Cambodian stretch of the river, took the record from a 293kg giant catfish that was caught upstream in northern Thailand in 2005.
The Mekong has the third-most diverse fish population in the world, according to its River Commission, although overfishing, pollution, saltwater intrusion and sediment depletion have caused stocks to plummet.
with reporting from AP
