Washington State Patrol Trooper Keith Leary said witnesses told investigators they saw the deputy lose control of the motorcycle as he entered the southbound lanes of the freeway from the Smokey Point Boulevard onramp.
"The next they know they saw the bike sliding sideways and tumble end-over-end and the deputy landing in the center of the freeway," Leary said.
Other deputies and state troopers rushed to the area near milepost 206 when the deputy did not answer radio calls.
Sheriff's spokeswoman Shari Ireton said the deputy was conscious when medics arrived.
He was airlifted to Harborview Medical Center in Seattle where he was listed in stable condition.
All southbound lanes of the freeway were blocked for several hours, creating a huge backup in the area.
Ireton said the deputy has been on patrol with the sheriff's office for about five years. His name has not been released.