A mother of two has been sexually assaulted in her Perth home while her children slept.

The 36-year-old woman was in bed at her home in Beeliar, in Perth's south, on Tuesday at 11pm (WST) when she was woken up by a man sexually assaulting her.

Police said the man had forced a window open to enter the home.

The offender demanded money from the woman and then forced her to drive him in her car to an ATM where she was forced to withdraw cash for him.

He then forced the woman to drive back to her house and ordered her to walk inside before fleeing.

Police say the incident is a reminder for the public to be aware of home security.

"People relax their security in the warmer months, leaving doors and windows open or unlocked, which gives offenders the opportunity to commit offences," a spokesman said.

"The public are reminded to keep their doors and windows locked ... "

The offender is described as aged between 20 and 25 years, with a very slim build, about 172cm tall, with a dark brown complexion, brown eyes, short-cropped black hair and a goatee beard.

He was wearing a navy T-shirt and cargo shorts and white socks.

Anyone with information is urged to call Crime Stoppers on 1800 333 000.