Firefighters from Bracknell, Wokingham, Windsor and Reading were called to Hornbuckle Farm in Drift Road at 4.25pm.
On arrival 30% of the 20m x 30m barn was alight, with stacks of hay bales on fire.
The crews set up two large water jets to attack the flames from the outside, before waiting for the contingency crews to take over due to the fire strikes from 5pm.
The contingency firefighters managed to drag some of the hay bales out of the barn, to prevent them from burning.
Asbestos in the roof was initially also a concern, but crews called it "low risk".
Firefighters left the scene at about 7pm, before returning again at 8pm to make a further assessment.
After concluding the scene was safe, they finally left at about 8.50pm.
Despite the barn being severely damaged, no animals or people were inside at the time.
There is no known cause of the fire but it is not deemed suspicious.