Four fire engines and around 25 firefighters were called to reports of a gas explosion on Garratt Lane in Earlsfield.
A ground floor flat in a three-storey block was damaged.
Firefighters rescued one man from the affected flat. He was taken to hospital where he later sadly died.
A man from an adjacent flat left before the Brigade arrived and was also taken to hospital.
The Brigade was called at 0726 and the incident was over for firefighters at 0946. Fire crews from Tooting, Croydon and Wimbledon fire stations were at the scene.
The cause of the explosion is under investigation by the Brigade and Metropolitan Police Service.