Bareilles was born and raised in Eureka, California. Her mother is Bonnie Halvorsen (née Capellas), a funeral home worker. Her father is Paul Bareilles, an insurance adjuster. Bareilles has Italian, German, Portuguese, and French descent. Bareilles was raised as a Catholic. She participated in the high school choir, Limited Edition, and local community theater musical productions, including her high school's production of "Little Shop of Horrors" as Audrey.