The RAC Rescue helicopter transferred the man to Royal Perth Hospital with two suspected broken legs after he crashed into a tree in an inaccessible area of bushland near Flynn Road and Great Eastern Highway about 11am.
A spokeswoman for RPH said he was in a stable condition.
It was the third serious accident involving off-road motorcycles last weekend.
A second man was injured when he crashed his motorcycle on Pinjarra-Williams Road near Dwellingup and a 40-year-old man died after he crashed an unlicensed trail bike on Montrose Avenue in Girrawheen on Friday night.
Quad bike deaths continue to rise. Last year, 23 people were killed in quad bike accidents nationally. So far this year, 13 have died.
Last week’s incidents come as new research shows just how risky it is to ride a motorcycle ” either off-road or on city streets.
A national study of 300 riders discovered that an alarming 45 per cent had been injured in motorcycle accidents.