California Highway Patrol Officer Robert Rodriguez said the semitruck was turning onto Muller Road from southbound Weedpatch Highway when the motorcycle, heading north on Weedpatch, crashed into the side of the truck's trailer.
The motorcyclist was identified as 22-year-old Joel C. Caraballo, of Lamont. He died at Kern Medical Center about three hours after the crash, the coroner's office said.
The truck driver, 57-year-old Kenneth E. Rasmussen, of Guerneville, wasn't hurt.
The Weedpatch-Muller intersection was closed for about an hour and a half.