The woman told sheriff's deputies she was walking in the Terrace Drive area when a man grabbed her, sexually assaulted her and then fled, sheriff's Sgt. Craig Scardina said.
The incident was reported at 10:41 p.m.
The victim was taken to a hospital for an examination, Scardino said.
The suspect is described as a black man between 18 and 25 years old who stands about 6 feet tall.
Anyone with information about the assault is asked to call the sheriff's office at (415) 499-7233 of Bay Area Crime Stoppers at (800) 222-TIPS.