The stop was made on Interstate 5 in the city of Carlsbad in San Diego County.
The officer spotted the 17-year-old girl in the backseat, tied up with a gag in her mouth.
According to the CHP, the teen admitted to using methamphetamine for the past year, and her father and 21-year-old sister had forced her into the car to take her to a treatment center in Mexico.
The teen was taken into protective custody.
No charges have been filed, but the case is under investigation.
The Associated Press contributed to this report.