Washington State Patrol Trooper Mark Francis said the man was hit by two pickup trucks about 5 a.m. and was taken to a hospital with life-threatening injuries.
Francis said the first driver saw the man dart out from his right. He swerved left, but struck the man with his passenger mirror.
"A couple of vehicles behind him was a Dodge Ram pickup truck," Francis said. "According to him, he sees that Ford F-150 swerve to the left, thinks his tire blew out, and that's when he looks up and sees pedestrian. He strikes the pedestrian full on."
Francis said neither driver appears to be at fault.
"We checked out the two drivers," Francis said. "No indication of alcohol or drugs... no charges against the drivers."
All southbound lanes of I-5 were blocked just north of SR-526 until about 7:15 a.m., creating a backup about 7 miles long.
It was not known why the man was on the freeway.