The largest freshwater fish in the world is the Mekong giant catfish. The Mekong giant catfish is a kind of giant catfish endemic to the Mekong River in Southeast Asia, with the largest individuals reaching 2.5-3 meters in length and weighing 200-300 kilograms. Native to the main Mekong River in Vietnam, Cambodia, Laos and Thailand, it also occurs in Myanmar and the Lancang River basin in southwestern China. In Thailand, it has been introduced into reservoirs and rivers, but neither has been able to establish a population. China was introduced in Thailand in 1986 for trial breeding in the south.

The largest freshwater fish ever caught by humans was a female Mekong giant catfish caught in northern Thailand in 2005, which weighed 293 kg and was 270 cm long. it is estimated to have a lifespan of 30 years. The giant bony tonguefish of the Amazon River basin in South America is also said to grow to more than 3 meters, but there are no clear records.

Scientists speculate that the survival of many aquatic giants has been seriously threatened by overfishing and destruction of their living environment. However, little is known about these fish because they generally live in dark, deep water in remote areas.

The Mekong giant catfish is considered the rarest fish in Southeast Asia and is currently endangered. The biggest threat to their survival is dams on the Mekong, where China has opened gates in the Lancang River basin in order to allow large ships to pass through, which has led to swifter currents and the destruction of the catfish's main breeding grounds.