The woman was driving northbound on I-5 near South 272nd Street at about 7:30 p.m. when her car was rear ended, causing it to burst into flames.
A WSP trooper in an unmarked car was also driving northbound on I-5 and spotted the fire, according to South King Fire and Rescue.
The trooper saw that the woman was trapped inside the burning car and used his radio to break a window and pull the woman out.
The woman was taken to Harborview Medical Center with non life-threatening injuries. Fire crews arrived soon after the rescue and extinguished the fire.
The trooper, who has not been identified, was not injured.