Dramatic images show the flames and smoke billowing from the top deck of the bus at around 7pm, having stopped stopped on Soho Road.
Fire crews extinguished the flames and no-one was reported injured at the scene.
A spokesperson for West Midlands Fire Service said: "Two crews from Handsworth fire station responded to Soho Road at 7.20pm on Wednesday (May 24).
"Firefighters wore breathing apparatus and used hose reels and a main jet to extinguish the fire, which is believed to have been started deliberately. They had left the scene by 8.30pm.
"No-one was reported injured."