Four fire engines and around 25 firefighters tackled a fire on Asylum Road in Peckham.
Part of the flat, in a four-storey house converted into flats, was damaged by fire. Three people left the property before firefighters arrived.
The Brigade was called at 2311 and the fire was extinguished by 0016. Crews from New Cross, Peckham, Old Kent Road and Deptford fire stations attended the scene.
The cause of the fire is under investigation.