Four fire engines and 21 firefighters and officers were called to a fire at a block of flats in Barcombe Avenue, Streatham.
Almost half of a six-roomed flat on the ground floor was damaged. One man left the property before the Brigade arrived. He was treated for smoke inhalation by London Ambulance Service crews.
The Brigade was called at 1312 and the fire was under control at 1409. Fire crews from West Norwood, Brixton and Tooting fire stations attended the scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.