The 33-year-old Pacifica woman was crossing San Pedro Road north of the Pedro Point Shopping Center when a driver heading south along the roadway hit her, according to police.
The woman was taken to San Francisco General Hospital with life-threatening injuries.
Police said officers at the scene determined the driver, identified as 32-year-old Robert Walton of Montara, was allegedly driving under the influence at the time of the crash.
He was arrested on suspicion of DUI and taken to San Mateo County Jail. The investigation is ongoing, and anyone who witnessed the accident is asked to call police at (650) 738-7314.