The suspect entered the woman's vehicle in the 1600 block of El Camino Real in Palo Alto and assaulted her, according to Palo Alto police.
The two then drove to the 400 block of Saratoga Avenue in Santa Clara, where the suspect sexually assaulted the victim.
The woman called Santa Clara police at 5:24 a.m. and flagged down a passing motorist, police said.
Santa Clara police handled the initial investigation, and the case was transferred to the Palo Alto Police Department.
The victim could not provide a suspect description, but police are asking anyone who saw her vehicle parked on El Camino Real or Saratoga Avenue Saturday morning to call Palo Alto police.
The vehicle is a black Mercedes Benz SUV.
Anyone with information about the incident can call the department at (650) 329-2413 or leave an anonymous message at (650) 383-8984. An anonymous text or email can also be sent to firstname.lastname@example.org