An emergency warning has been issued for parts of Aubin Grove and Hammond Park, about 30km south of the city centre.
A number of roads have been closed including part of the busy Kwinana Freeway, the Department of Fire and Emergency Services said on Wednesday.
"You are in danger and need to act immediately to survive," the warning said.
"Homes in the area are under threat by fire now."
People in the area have been told to leave immediately if it is safe to do so or find shelter if they cannot.
"Go to a room in your home away from the fire front and make sure you can easily escape," emergency services said.
"Choose a room with two exits and water such as a kitchen or laundry."
The fire started near the intersection of the Kwinana Freeway and Rowley Rd in Aubin Grove.
People in the area should leave west along Russell Rd or north along Hammond Rd if it is safe to do so.
Western Australia is currently in the grips of a heatwave, with Perth forecast to reach 39C on Wednesday.
The mercury hit 36C at 12.50pm local time, with gusty northeasterly winds.
17°C