Valerie Calderon Casillas was last seen entering a blue SUV around 3 a.m. Sunday near Waverly Avenue and Orlando Drive.
She was reported missing later that day at about 7:15 p.m.
Police have not determined the vehicle's license plate number or the identities of its occupants.
Casillas is described as a Hispanic woman with brown hair and brown eyes and medium complexion. She stands at 5 feet 3 inches tall and weighs 140 pounds.
Anyone with information about Casillas' whereabouts is asked to contact Detective Sgt. Eric Quan at (408) 277-5250. Anyone wishing to remain anonymous may call Silicon Valley Crime Stoppers at (408) 947-STOP (7867) or visit www.svcrimestoppers.org.