Two men have been arrested for allegedly stabbing to death their cousin in Zondi, Soweto police said on Sunday.

Inspector Kay Makhubela said the teenage boy was killed when his older cousins tried to stop him from assaulting his mother on Saturday night.

"The boy was beating up his mother and when the two cousins tried to intervene they ended up stabbing him with a sharp object on the chest, killing him on the scene," Makhubela said.

It was not yet known why the teenage boy was assaulting his mother.

The two men aged 19 and 30 are expected to appear in the Protea Magistrate's Court on Monday, on charges of murder.