State Patrol spokesman Trooper Keith Leary said the trooper was southbound on I-5 and hit the man near the ramp to East College Way at about 9:45.
The man was apparently attempting to walk across the highway, Leary said, calling the situation a tragic accident.
Leary said the trooper was shaken but not injured.
Troopers closed the ramp to East College Way for several hours while they collected evidence at the scene. The investigation is being handled by both the State Patrol and the Mount Vernon Police Department.
The name of the man who was killed has not been released.
"Our hearts go out to that family," Leary said.