Alcohol and excessive speed may have been factors in the crash, troopers said.
State troopers responded to the scene, the exit from southbound I-5 to 6th Avenue South, at about 11:50 p.m. after receiving a report of a serious crash.
A preliminary investigation found that the car was heading south on I-5 when it crashed into the front of the barrier at the 6th Avenue off-ramp, then hit the right side barrier.
A young woman riding in the back seat was ejected from the car and killed. It does not appear that she was wearing a seat belt, troopers said.
The driver and four other passengers were injured. All were taken to Harborview Medical Center.
The State Patrol says it is investigating the possibility of vehicular homicide charges against the driver.
Anyone who witnessed the crash is asked to contact the Washington State Patrol.