Main Roads has also closed South Western Highway northbound, south of Boyanup.
A man in his 20s is being rushed to Royal Perth Hospital by the RAC rescue helicopter after being hit by a truck.
Around 7:30 this morning the truck struck a pedestrian at the South Western Highway and Shenton Road intersection in Boyanup.
The victim sustained head and back injuries in the crash.
Police remain on site with traffic management in place.
Main Roads advises motorist to seek an alternate route.