The stingray is more than twice the length of an average human and weighed just under 300 kilograms.
It was caught in the Mekong River which runs through most of South East Asia.
The fish was caught by a local fisherman and the record was recorded by the Wonders of the Mekong, a joint Cambodian-US research project.
The previous record for a freshwater fish was a 293-kilogram (646-pound) Mekong giant catfish, discovered in Thailand in 2005, the group said.
The scientists arrived within hours of getting a post-midnight call with the news of the giant fish being caught and were amazed at what they saw.
“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,” Wonders of the Mekong leader Zeb Hogan said.
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.
"The fact that the fish can still get this big is a hopeful sign for the Mekong River," Mr Hogan said, noting that the waterway faces many environmental challenges.
The Mekong River runs through China, Myanmar, Laos, Thailand, Cambodia and Vietnam. It is home to several species of giant freshwater fish but environmental pressures are rising. In particular, scientists fear a major program of dam building in recent years may be seriously disrupting spawning grounds.
"Big fish globally are endangered. They’re high-value species. They take a long time to mature. So if they’re fished before they mature, they don’t have a chance to reproduce," Mr Hogan said.
"A lot of these big fish are migratory, so they need large areas to survive. They’re impacted by things like habitat fragmentation from dams, obviously impacted by overfishing.
"So about 70% of giant freshwater fish globally are threatened with extinction, and all of the Mekong species."
The team that rushed to the site inserted a tagging device near the tail of the mighty fish before releasing it.
The device will send tracking information for the next year, providing unprecedented data on the largely unknown behaviour of giant stingrays.
Local residents nicknamed the stingray "Boramy,” or “full moon,” because of its round shape and because the moon was on the horizon when it was freed on June 14.
In addition to the honour of having caught the record-breaker, the lucky fisherman was compensated at market rate, meaning he received a payment of around $600 (£490).