The woman was found lying in the westbound lane at the intersection of Apsley Road and Brolga Promenade around 12.30am on Wednesday.
She was rushed to Fiona Stanley Hospital but died from her injuries soon after arriving.
A WA Police spokeswoman said it was not clear whether the woman had been hit by a car or sustained her injuries elsewhere.
The woman has dark skin and has not yet been identified.
Anyone who was in the area around Apsley Road and Brolga Promenade around the time of the incident, or anyone with any information is asked to call Crime Stoppers on 1800 333 000 or report the information online atwww.crimestopperswa.com.au