Aranza Maria Ochoa Lopez is believed to have been abducted on Thursday by her non-custodial mother, Esmeralda Lopez, 21.
Police believe Lopez might be taking her to Mexico.
California Highway Patrol issued the Amber Alert this morning, believing the pair may be passing through California to the Mexico border.
Lopez took the girl during a supervised visit in Vancouver, Washington, near the Oregon state line. Four-year-old Aranza has been in foster care since last year and is desribed as having blond hair and blue eyes.
Lopez is described as Hispanic, 5 feet tall and weighing 138 pounds. She has black hair and brown eyes.
Lopez was last seen driving a red 2005 Chevy Cobalt with the Washington license plate BLK 1552.