Crews from West Midlands Fire Service stations on Bilston and Willenhall were called to reports of a double decker bus on fire near the Malthouse pub on New Road, Willenhall.
The fire, which was reported at 10.05pm on Monday night, started on the top deck of the 529 National Express service between Wolverhampton and Walsall and all people on the bus had been evacuated by the arrival of the crews.
Ten firefighters worked to extinguish the fire, with a National Express mechanic ensuring the vehicle's electrics were made safe afterwards.
No injuries were reported at the scene, with fire crews saying they believed the incident, which destroyed 80 percent of the top deck of the bus, to be an act of arson.
A spokesman for West Midlands Fire Service said: "Crews from Willenhall and Bilston, 10 firefighters on two fire engines, were called at 10:05pm to reports of a double decker bus on fire near The Malthouse, New Road, Willenhall.
"The incident involved a fire on the top deck of the bus, which was 80 per cent destroyed by fire.
"All persons had left the vehicle prior to the arrival of the fire service and no-one suffered any injuries.
"Our crews extinguished the fire, and the vehicle’s electronics were made safe by a National Express mechanic that attended the scene.
"Our crews do believe the incident to be arson. Police were also in attendance."
West Midlands Police and National Express have both been approached for a comment.