Briella, who goes by the name Luna, was last seen late Sunday night after she left her home in the Lush Meadows area in Mariposa County.
At the time, she was wearing pajamas and she may be carrying a black backpack.
She's 5 feet 3 inches tall and weighs about 96 pounds with blue eyes and brown hair.
If you have any information, please call the Mariposa County Sheriff's office.