It happened just after 7 p.m. Monday at the intersection of Berry and Washington Streets in Calistoga.
CHP says a police pickup truck hit a 22-year-old woman who was crossing the street, but it's unclear if she was walking in the crosswalk.
She was taken to the hospital to be treated for what police are calling major head injuries.
Officers are investigating if the patrol car had its sirens and lights on.
No one else was hurt and the road has since reopened.
There's no word yet on any disciplinary action against the officer driving the police pickup truck.