The blaze needed 57 firefighters to bring it under control after it broke out in the early hours of 28 December at First Call Coaches in Pant Industrial Estate, Merthyr Tydfil.
South Wales Fire and Rescue Service confirmed it was treating the blaze as a "deliberate fire".
Investigators studied CCTV footage to help decide the cause.
The enormous blaze triggered a series of explosions.
The coaches were parked very close together, partly as a security measure to prevent anyone trying to steal fuel or batteries and parts.
However, firefighters said that had helped the fire spread quickly.
Describing the scene, Dewi Jones from South Wales Fire Service told BBC Wales: "There's total devastation.
"Prompt action prevented a fire from spreading to surrounding businesses."
A huge fire at a bus depot which destroyed 30 coaches, five minibuses and a car is being treated as arson.