The toddler, identified by the medical examiner's office as Mi'yana Gregory, of San Francisco, was struck as she crossed Mission Street in a crosswalk with a family member between Fourth and Fifth streets, police said.
The vehicle, which was traveling east when it hit the girl, is described as a white mid-1990s four-door sedan, possibly a Toyota Camry, Honda or Lexus, with a dark-colored mismatched front bumper and dark or black rims, police said.
On Sunday night, the family held a vigil at the accident site.
Anyone with information about the case is asked to call the Police Department's anonymous tip line at (415) 575-4444 or to send a tip by text message to TIP411 with "SFPD" in the message.
ABC7 News contributed to this story.