A 20-year-old man was arrested for allegedly having sex with the family dog on Thursday, Gauteng police said.

The man's sister walked in and witnessed the crime at their home in Katlehong, Sergeant Obakeng Tsomane said.

She told her parents about it who went to the police. The dog belonged to the family and had been in their care for three years.

It was not known what breed it was.

The man was arrested and charged with bestiality. He was due to appear in the Alberton Magistrate's Court on Friday.