The incident began at about 1:30 a.m. when several motorists called 911 to report a man walking or running along I-5 near mid-Boeing Field.
State troopers responded to the area to look for the man, but before they could locate him he was struck by a 2010 Hyundai Sonata traveling south at about 60 mph. The impact shattered the front windshield of the car.
The victim, identified as a 47-year-old Seattle man, had only a faint pulse when medics arrived at the scene. He was taken to Harborview Medical Center, where he later died of his injuries.
The driver of the car, a 46-year-old Auburn man, was very shaken up but sustained only a minor injury to his hand. He was not under the influence of alcohol or drugs and no charges will be filed, the State Patrol reports.
Two southbound lanes of Interstate 5 were closed for about two hours as state troopers investigated the crash.