The bus was on Geary Boulevard near 28th Avenue when the back of the bus started burning around 3:10 p.m., fire spokeswoman Mindy Talmadge said.
An engine problem apparently led to the blaze, she said.
All passengers and the driver were off the bus by the time fire broke out, she said.
Firefighters responded and put out the blaze. No injuries were reported, she said.
The 38-Geary Muni bus runs along Geary Boulevard.