The bus was driving near the intersection of MacArthur Boulevard and Ardley Avenue at about 5:10 p.m. when the man was set on fire, police said.
AC Transit spokesman Clarence Johnson said the victim was taken to the burn center at St. Francis Memorial Hospital in San Francisco. Neither police nor Johnson could disclose the severity of his injuries.
Johnson said the driver of the Line 57 bus noticed the man on fire but did not know how it happened. The driver stopped the bus to call for assistance and several people exited the back of the bus.