Federal Aviation Administration spokesman Ian Gregor said the pilot reported engine trouble before bringing the single-engine Piper PA-46 down on a grassy strip.
Gregor said the pilot, who was the only person on board, wasn't hurt during the landing near San Martin Airport, south of San Jose. The CHP said there were no injuries on the ground.
Twitter user Charlene Nunes shared a picture and video from the highway as officials responded to the landing.
No further information was available on what may have caused the plane to land on the freeway.
The Associated Press contributed to this report.