Chilling footage has emerged of a four-year-old boy being thrown off a bridge in the US while his mother cheered in the background.

The video shows the boy plummeting eight metres after the unnamed woman's boyfriend tossed the child off a Devonshire railroad bridge in Montesano, Washington.

"This is what happens when some parents just don't care," Kaylub Fawley, who witnessed the incident, captioned the video on Facebook.

"This is a woman me and all my friends witnessed letting a 'stranger' throw her 4 1/2 year old son off the bridge into the water for fun!"

Posted by Kaylub Fawley
Posted by Kaylub Fawley
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Fawley watched on as the child plunged into the water that was filled with large underwater boulders.

"I looked up and was like, 'Whoa, there's a kid up there' - I immediately started recording," the 18-year-old told The Washington Post.

"This guy just threw the kid right off the bridge, and he smacked down right on his neck."

The child's lifejacket eventually pulled him to the surface of the river

Police offers arrived 15 minutes after the incident and the boy was assessed by paramedics.

The boy is reportedly fine but according to Fawley "he was shaken up pretty bad".

Prosecutors are investigating the pair but have not yet decided whether to charge the mother with child endangerment, reports The Washington Post.