The buses were parked in the garage of the Urubupunga company when a group of people went inside and set fire to the vehicles, most of which were totally destroyed.
Sao Paulo state police said over a score of buses were set ablaze, adding that no one has been reported injured. No arrests have been made.
The buses that were burned represent 20 percent of the fleet of 178 vehicles that served 21 lines in the city, mostly in the northern suburb of Osasco, 16 km from Sao Paulo city.
In recent months, buses have been attacked in greater Sao Paulo as part of the demonstrations demanding better public services in the city's poor areas and by others protesting the death of a person in a police operation. But many have simply been the target of vandals.