The robbery was reported at about 7:55 p.m. aboard a 5-Fulton bus near McAllister and Gough streets.
The woman got into an argument with two men on the bus, and one grabbed and punched her and the other snatched her phone and briefcase, according to police.
One of the suspects then threw the woman down and the pair fled from the bus. Described as in their 20s, the men were last seen fleeing in a black, four-door vehicle, police said.
The woman was not injured in the robbery.
Anyone with information about the case is asked to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411 with "SFPD" in the message.