This is the dramatic moment a hero passersby rescued a boy drowning in a river in Thailand.

The 12-year-old lad had been fishing with his friends on the banks of the Chao Phraya River in the Phra Pradaeng district of Samut Prakan province on the outskirts of Bangkok when he slipped and fell into the water on October 1.

The currents had swept him several metres away from dry land when a quick-thinking courier named Yim, 47, who heard the cries for help while walking through the area, rushed over to save his life.

Dramatic footage shows the hero local in a black shirt and pants fearlessly plunging into the river to paddle toward the struggling youngster seen dipping in and out of the water.

Tense onlookers applauded when the man finally reached the drowning boy and handed him the lifebuoy.

Still carrying the lad in one arm, the rescuer swam back toward the pier, where they were hauled up by relieved bystanders.

A resident named Wichian, 52, said locals often fished at that section of the dock, but there was a lot of moss that can cause them to slip into the river.

Onlooker Janie said: 'The child fell into the water and the hero jumped in to help without any hesitation. He put the boy's life first to rescue them.'

The boy has been safely reunited with his family and did not suffer serious injuries.