The fire happened above Candy Love, in Montague Street, at around 12pm and the man, thought to be middle-aged, was pulled out at about 12.20pm.
The man has been taken to hospital because of smoke inhalation.
Candy Love owner Paul O’Brien called the fire brigade when a member of staff phoned to tell him the shop’s alarms were going off.
He said: “There was thick black smoke and from what I could see, it was pretty bad.
“I called the fire brigade and managed to get another couple out of the building.
“They just pulled someone out but I don’t know how bad it is yet.”
Roy Barraclough, of West Sussex Fire and Rescue Service said: “We were called at midday and pulled a man out of the second-floor flat.
“He is being treated for smoke inhalation. We are investigating the scene now.”
The scene has been cornered-off and large crowds had gathered at the scene.