The Cypress Police Department said the woman was using the bathroom just after 3 p.m. Monday when she saw a hand reach under the stall divider and point a cellphone at her, the Los Angeles Times reported Friday.
The woman chased a man she saw fleeing from the adjacent stall and called police while running.
"She was calling us on the phone as she's running after the guy," officer Paulo Morales said. "She had her wits about her."
The suspect was able to elude the woman and arriving officers when he jumped a wall and made it to the busy Lincoln Avenue.
The suspect is described as Latino, about 200 pounds and 20 to 25 years old. The woman said he had dark hair with the sides of his head shaved.