State troopers responded to the scene, on I-90 about five miles west of George, at 9:35 p.m. Friday after receiving a report of an accident.
The woman, identified as Sheri A. Warner, 35, of Beaverton, Ore., was found dead at the scene.
The motorcyclist, a 56-year-old Federal Way man, was not injured. Drugs or alcohol were believed to be a factor in the accident, and he faces possible charges of DUI.
A preliminary investigation found that the black 2001 Harley-Davidson FXSTD motorcycle was headed east on I-90 when the passenger was somehow ejected and landed in the left lane. She was struck by another vehicle that kept going.