Firefighters from Bracknell and Wokingham stations rushed to Crowthorne Road at 8pm to find the empty Adastron House, near the junction with Downshire Way, fiercely ablaze.
The crews spent about two-and-a-half hours extinguishing the flames, which were primarily in two of rooms.
They checked that there were no homeless people inside the building and found it was clear.
One crew member said: "It's just a mess in there, but the building is structurally sound. One of the rooms has a hole in the floor and the damage was contained to those two rooms."
Four firefighters with breathing apparatus and one hose reel was used.
Police were also in attendance as it is believed the fire was started deliberately.