Emergency services were called to a report of a fire at Pizza Hot Express in Brighton Road, East Worthing, on Wednesday (March 18) at 9.15pm.
West Sussex Fire and Rescue put out the blaze which caused extensive damage to the property and police cordoned off the road, police said.
No one was hurt, according to police.
A police spokesman added: "An investigation into the cause of the fire has started. Any witnesses or anyone with information about the fire should report online or ring 101 quoting serial 1167 of 18/03."
Crews from Worthing, Lancing, and Bognor Regis were called to the blaze at about 10pm.
Firefighters used four sets of breathing apparatus, two hose reels, one jet and a hydrant to tackle the fire in the kitchen of the first floor flat.
All residents were accounted for, according to West Sussex Fire and Rescue Service (WSFRS).