Mason County sheriff's deputies and medics responded to the scene, on Cloquallum Road just west of the Shelton Valley Road, at about 4:30 p.m.
The passenger was thrown from the motorcycle as it slammed into a guardrail. She was pronounced dead at the scene.
The driver sustained multiple broken bones and was airlifted to Harborview Medical Center in serious but stable condition.
An investigation is under way to determine if drugs or alcohol were a factor in the crash.
The names, ages and hometowns of the driver and passenger were not immediately available.