Four fire engines and around 25 firefighters tackled the blaze in Bramley Hill at 3.40am yesterday (June 19).
Firefighters led a man to safety and a further 30 people left the building before the brigade arrived.
He was taken to hospital by London Ambulance Service crews where he sustained serious injuries.
Officers are investigating the circumstances around the fire which is currently being treated as arson.
No arrests have been made as police enquiries continue.
Crews from Croydon, Wallington and Woodside fire stations attended the scene.
The fire was under control just after 5.20am.
A spokesperson for Metropolitan Police said: “Police were called 03:48hrs on Thursday, 19 June to Bramley Hill, South Croydon following reports of a fire.
“One person remains in hospital after sustaining serious injuries.
“Officers are investigating the circumstances around the fire which is currently being treated as arson.
“No arrests have been made at this stage and enquiries are ongoing.”
