Police are searching for a man who sexually assaulted a 23-year-old woman in her home in South Fremantle overnight.

The woman woke early on Saturday morning to find a male intruder armed with a knife in the bedroom of her Wesley Street home.

He is described as being 17 to 18 years of age, 165 centimetres tall, and of slim build. He was wearing a blue t-shirt.

Police are urging all residents in the area to check their backyards and bins for any unusual items such as clothing that may be there.

They are also asking anyone with information on the attack to come forward.