The robbery occurred at about 9 p.m. near the intersection of Mission Street and Richland Avenue.
The 46-year-old woman was sitting on the bus with her purse on her lap when a suspect approached and took it from her, according to police.
The victim tried to hold onto her purse but was overpowered by the suspect, a male between 16 and 20 years old who then got off the bus and ran away, police said.
The suspect had not been found as of Friday morning.
Anyone with information about the robbery is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.