This is the moment a heroic passerby rescued a drowning father trying to save his son from a river in Thailand.

Somrerk Sun, 26, was sitting on the banks of the Ping River when he heard the man screaming for help in Kamphaeng Phet province on March 30.

Despite a red sign warning people not to swim, fearless Somrerk plunged into the water and paddled toward the local who was struggling to get to safety.

He managed to drag the exhausted father back to shallow waters as onlookers rushed over to pull them out of the river.

Somrerk told local media: 'I thought it was a joke at first until I saw the man saving his drowning son. They were both swept into a deeper part of the river. Another person saved the son, but the father still needed help so I jumped in.

'I would like to warn visitors not to play outside the designated swimming area. This river is dangerous and several people have drowned before.'