Four fire engines and around 25 firefighters tackled a fire on Union Square in Battersea.
Part of a sauna room on the first floor of a 13-storey building, made up of flats and a leisure suite, was damaged by fire. There were no reports of any injuries.
The Brigade was called at 0635 and the fire was brought under control by 0743. Crews from Battersea, Lambeth and Brixton fire stations attended the scene.
The fire is being treated as accidental and is believed to have been caused by combustible items being too close to a heat source.