A man has received life-threatening injuries are being hit on the Kwinana Freeway in Baldivis.
Around 9pm on Thursday, a Renault Clio hatchback was travelling southbound along the freeway between Karnup Road and Paganoni Road, when it collided with a pedestrian.
The pedestrian, a man in his 20s, was critically injured and is now in Royal Perth Hospital.
The driver of the vehicle was uninjured and is assisting police with their inquiries.
Investigators would like to speak with anyone who may have dash-cam footage of Renault Clio driving in the area, or the pedestrian walking in the area, around the time of the crash.
Anyone with any information is asked to contact Crime Stoppers on 1800 333 000 or report online at Crimestopperswa.com.au.