The back of the 30X-Marina Express bus was reported on fire around 8:45 a.m. at a stop at Howard and Main streets, Muni spokesman Paul Rose said.
Muni crews quickly put out the fire, which started in the engine compartment in the back of the bus, Rose said.
There were no passengers onboard and the driver was able to get out safely, Rose said.
The last passengers had gotten off at the previous stop on the inbound route.
Rose said the bus is an older-model coach that has been towed to a bus yard to evaluate the damage and cause of the blaze.