The Redwood City girl was riding at the Clermont Equestrian Center at Cypress Ridge on Skyline Boulevard in Woodside when she fell from a horse.
The girl was in the middle of a riding lesson and had ridden the horse six times before without any problems, deputies said.
During the lesson, the horse spooked for an unknown reason, bucked her off and she fell to the ground.
During her fall, the little girl was dragged by the horse for several seconds and kicked in the head. The girl suffered severe injuries.
Witnesses, which included the victim's trainer and her grandmother, called 911 immediately and provided CPR and other life-saving efforts until paramedics arrived.
An ambulance brought the girl to a helicopter, which brought the girl to Stanford Hospital, where she died.
Deputies have not released the name of the girl.