Six fire engines and around 40 firefighters were called to a fire at a flat on Deals Gateway, Deptford.
Watch Manager Matt Cole, who was at the scene, said: "The alarm was raised when the automatic fire detection system fitted throughout the building's communal areas flashed on a control panel at ground floor level. Reception staff went to investigate, saw smoke and immediately called 999.
"Firefighters wearing breathing apparatus searched the affected flat and found it empty but rescued a cat. They gave it oxygen and it was later taken to a local vet by neighbours."
A small part of a fifth floor flat was damaged by fire. Around 30 residents left the building before the arrival of the Brigade.
The Brigade was called at 1345 and the fire was under control at 1517. Fire crews from Deptford, New Cross, Lewisham, East Greenwich, and Lee Green fire stations attended the scene.
The cause of the fire is believed to be accidental.