Ashley Barraza, 18, was dropped off near her school on Monday but never made it to class.
Family members said she recently met a man online with whom she spent the whole day.
The man told detectives he dropped her off at one of her friend's houses located at Garvey and Garfield avenues in Monterey Park.
Barraza is described as Hispanic with black hair, brown eyes, 5 feet 5 inches tall, weighing approximately 190 pounds. She was last seen wearing a black T-shirt, blue jean shorts, black tennis shoes and was carrying a green backpack.
Anyone with information regarding Barazza's whereabouts is asked to call (877) LAPD-24-7 or (877) 527-3247.
City News Service contributed to this report.