Police are investigating whether there was a malicious motive behind the stretch of steel.
The 16-year-old boy and a friend were both riding motorbikes on a Pinjar Road property about 3.45pm.
The victim was travelling on the southern side of the property next to a strawberry farm when he was flung from his seat. The wire had been attached to the strawberry farm fence and pulled taut across the track.
The boy was treated in Sir Charles Gairdner Hospital for serious injuries.
Anyone with information should contact Crime Stoppers on 1800 333 000.