The girl, identified as Jasmine Martinez, died on Saturday, according to the coroner's office.
Martinez was found unconscious after riding the Revolution roller coaster at the amusement park on Friday.
Firefighters were called to the park shortly after 4:00 p.m. and said the girl was airlifted to the hospital.
It's unknown if her death was ride-related. An autopsy is pending to determine the cause of death.
Six Flags released a statement, extending condolences to the girls' family and friends.
We are deeply saddened to learn of the death of Jasmine Martinez. Our thoughts and prayers are with her family. http://t.co/JtEYWS7WVe— Six Flags MM (@SFMagicMountain) June 15, 2015
Also in the statement, park officials said more than 45 million guests have safely ridden the Revolution roller coaster since it opened in 1976.