The woman was walking on the Centennial Way Trail between South Spruce and Tanforan avenues shortly after 5 p.m. when she encountered the man. He called out to her, at which point she looked over and noticed he was masturbating, police said.
The woman immediately walked away and contacted police, and the suspect was last seen driving off on South Maple Avenue, police said.
The suspect is described as a white man about 25 years old, 5 feet 9 inches tall, clean-shaven with a medium build and short blond hair. He was driving a black, four-door compact car similar to an early 1990s Toyota or Honda, police said.
Anyone with information about the incident is encouraged to call South San Francisco police at (650) 877-8900.
