Sussex Police said they were called to Brighton Road at around 7pm after the fire in top floor of the building, near to the junction with Arthur Road.
Fire crews from West Sussex Fire and Rescue were called to help tackle the blaze late in the night, forcing the road to be closed.
Following the incident, the force confirmed that a man had been arrested and remained in custody as this time.
A spokesperson for Sussex Police said: "Officers arrested a 23-year-old man from Horsham on suspicion of arson with intent to endanger life and criminal damage.
"He has been taken to custody, where he remains at this time.
"Anyone who witnessed the incident or has information to assist police enquiries is asked to make a report. You can do this online, or by calling 101 quoting reference 1076 of 26/11.”
A spokesperson for West Sussex Fire and Rescue added: “Yesterday (November 26) at 7.13pm we responded to a fire at a first floor flat in Brighton Road, Horsham.
"Joint Fire Control sent three fire engines from Horsham and Crawley to the scene.
"Upon arrival firefighters wearing breathing apparatus entered the property and extinguished the fire using hose reels and jets.
"Fortunately there were no casualties and everyone was accounted for.
"The fire is believed to be of deliberate ignition an investigation is now being carried out by Sussex Police."
1. Man arrested on suspicion of arson following fire at flat in Horsham
Man arrested on suspicion of arson following fire at flat in Horsham Photo: Eddie Mitchell
2. Man arrested on suspicion of arson following fire at flat in Horsham
Man arrested on suspicion of arson following fire at flat in Horsham Photo: Eddie Mitchell
3. Man arrested on suspicion of arson following fire at flat in Horsham
Man arrested on suspicion of arson following fire at flat in Horsham Photo: Eddie Mitchell