The man was struck and injured by an outbound 14 Mission bus at Fifth and Mission streets around 12:20 p.m., said Muni spokesman Paul Rose.
The bus was just pulling away from the stop at the time of the incident, and the pedestrian appears to have been crossing the street in front of the bus, Rose said.
He was taken to a hospital for treatment but died of his injuries. The pedestrian was taken to a hospital for treatment of his injuries but has died, according to Rose.
Ed Reiskin, director of transportation for the San Francisco Municipal Transportation Agency, expressed the agency's "deepest sympathy" for the family and friends of the pedestrian.
"The SFMTA will continue to work with the SFPD as we continue to investigate the details of this incident," Reiskin said.
The driver has been placed on leave pending an investigation, and will be tested for drugs and alcohol according to Muni protocol, Rose said.