Deputies responded to a reported of an unconscious girl in the community of Agua Caliente near Sonoma at about 10:30 p.m., according to the Sonoma County Sheriff's Office.
An ambulance took the girl to a hospital, where she was pronounced dead shortly after arriving, the sheriff's office said. The cause of death has not been determined.
Investigators do not suspect foul play and are investigating the incident.
It has not been determined if drugs or alcohol were a factor in the girl's death.