A 28-year-old woman was walking past Forrestfield Forum and Market Place shopping centre about 6.50am on December 14 when she heard a vehicle pull up behind her.
A male voice shouted out something she did not understand.
She turned around to see a man approaching her with a large knife in his hand.
He demanded cash and when the woman took money out of her purse, he snatched it and ran to a gold Toyota Camry sedan.
The man was dark skinned, aged 20 to 25 with short brown hair, facial hair, about 160-165cm tall and of medium build.
He was wearing dark coloured pants with a light stripe down the leg.
Call Crime Stoppers on 1800 333 000, or report online at www.crimestoppers|wa.com.au.