Four fire engines and around 25 firefighters were called to a fire on Loughborough Road in Brixton.
Part of the maisonette on the third and fourth floor was damaged by the blaze.
A woman and a child left the property, uninjured, before the Brigade arrived. Firefighters rescued a dog from the maisonette.
Station Commander Paul Sadler, who was at the scene said:
"Two firefighters wearing breathing apparatus found the dog laying on the floor at the top of the stairs. The Chihuahua called Chopper was brought out of the property and fire crews started giving chest compressions and administered oxygen.
"Chopper thankfully came round and was taken to the vets for a precautionary check up. The firefighter's actions undoubtedly saved the dog's life and he is now doing ok."
The Brigade was called at 1115 and the fire was under control by 1154. Fire crews from Clapham and Old Kent Road fire stations were at the scene.
The cause of the fire is under investigation.