The Ada County Sheriff's Office says the 16-year-old Eagle girl had been hospitalized since the crash, but never regained consciousness.
Her identity is not being released at this time.
The crash occurred on the evening of Monday, December 10 at the intersection of Highway 16 and Floating Feather Road.
Officials say a Jeep Cherokee being driven by a 50-year-old woman from Emmett was traveling north on Highway 16.
Another car, with a 16-year-old boy from Star and the 16-year-old girl from Eagle, was traveling west on Floating Feather and tried to cross Highway 16, when the two vehicles collided.
The teenage male's injuries were considered serious but not life threatening.
An air ambulance was sent to the scene to fly the older women to St. Alphonsus Hospital in Boise where she was listed in serious condition.
All three were wearing seatbelts, and airbags deployed in both vehicles.