The flames broke out at the Landmark Diner on South Highland Avenue in Briarcliff Manor around 10 a.m.
Streets around the restaurant were closed as firefighters dealt with hot spots.
No injuries were reported, but the building suffered significant damage.
The Ossining Police Department posted about the fire on Facebook, calling it "a sad day in Ossining."
The fire department says the fire started in the kitchen and quickly spread to the roof, and building construction made it difficult to extinguish the fire.
Firefighters were unable to enter the diner and needed to fight the flames from outside.
Two volunteer firefighters suffered minor heat-related injuries.
The cause of fire is under investigation.
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