The crash was reported at 1:20 p.m. on southbound U.S. Highway 101 just north of Spencer Avenue.
The motorcyclist suffered fractures and bleeding and was non-responsive when he was transported by ambulance to Marin General Hospital, CHP Officer John Short said.
The No. 4 lane was blocked while crews worked to clear the wreckage, but was expected to be cleared by about 2:30 p.m., Short said.
