Police want to speak to a man they have described as "extremely dangerous" in relation to the discovery of a woman's body in a car on Perth's Mitchell Freeway early on Saturday.

The woman has been identified as 35-year-old mother of three Rebecca Jane Gascoigne, who lived in Scarborough.

Detectives are searching for Shawn Adam Newton, 42, who is also wanted for questioning over an armed robbery at a pharmacy, Craigie, on July 7.

Detective Senior Sergeant Dean Bailey is urging anyone who sees the man to call police.

Rebecca Gascoigne's body was found in a car on Perth's Mitchell Freeway. ( Supplied: WA Police )

"He is considered extremely dangerous and the public is warned not to approach him as he is in possession of a firearm," he said.

"Shawn Newton is described as being fair skinned, around 186 centimetres tall, medium build, very short dark hair with brown eyes."

Police discovered Ms Gascoigne's body in a car on the Mitchell Freeway in Stirling just after midnight on Saturday.

A 49-year-old man approached police on the Cedric Street off-ramp while they were attending to a car crash, to tell them he had a body in his car and that he was being chased.

That man is assisting police with their inquires.

Police said the man, Ms Gascoigne and Mr Newton are all known to each other.

Police also searched two houses in Scarborough on Saturday in connection with the investigation.