Fire crews were called to Côte Brasserie, in East Street, Horsham, at 3.06am to reports of the fire.
Six fire engines were sent to the scene before the fire was brought under control at 07.13am, according to WSFRS.
A spokesman for WSFRS said: “A call came in relating to a fire in a two-storey building, with one of the floors being used as a restaurant.
“Around 25 per cent of the restaurant has been damaged by fire and smoke.
“The roof to the building has been 100 per cent damaged by both smoke and fire.”
There were not thought to be any people in the building when the fire broke, WSFRS said.
The cause of the fire is currently unknown as fire crews, assisted by police, remain on scene to investigate the cause.
The WSFRS spokesman added: “Six breathing apparatus were used, as well as three jets, one safety jet, one 13 and a half metre ladder and three high pressure hose reels.”
WSFRS was unable to confirm whether there was any damage caused to surrounding buildings.
Fire crews remain on the scene.