Police say the woman, who is in her 80s, was hit by a vehicle in the 1400 block of South 312th Street at roughly 5:20 p.m.
She was taken to the Harborview with life-threatening injuries.
The driver was not injured and remained at the scene until police arrived. The Federal Way Major Accident Investigation Team is looking into the crash, but so far nothing indicates the driver was under the influence of drugs or alcohol, according to police.
Westbound South 312th Street is closed to traffic between Pacific Highway South and 14th Avenue South while police investigate the crash.