The decomposing body of an unknown man was found floating in a river in Sweetwaters in the Vaal Rand on Tuesday evening, police said.

It looked like it had been in the water for a long time because it was starting to decompose and some body parts looked like they had been eaten by crabs, said Constable Tikoane Sonopo.

"Sweetwaters residents saw the naked body and contacted the police around 6pm.

"It was difficult to estimate the man's age because of the state of the decomposing body," said Sonopo.

He said it was unclear whether the man was killed and thrown into the river, or he just drowned.

An inquest docket was opened.