A young motorcycle rider has died in a horrific crash south of Perth after he was struck while stopped in an emergency lane.

WA police say the crash took place in the emergency lane on the Forrest Highway in West Coolup, southwest of Pinjarra, about 10.45pm on Friday.

The 20-year-old Yamaha rider pulled over to a northbound emergency lane about 200m south of the intersection with Hill Road.

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A white Toyota Land Cruiser was also driving north on the same section of the highway.

Police say a crash involving the two vehicles occurred in the emergency lane, killing the 20-year-old.

Anyone who saw the accident, or the white Toyota Land Cruiser or Yamaha motorcycle before the crash, are asked to contact police.

There have been 148 deaths of WA roads so far this year.