The 16-year-old boy is in a critical condition and has been taken to a nearby hospital.
Scotland Yard said officers were called to the scene on York Road, Wandsworth, at about 12.45am on Sunday.
No arrests have been made.
Wandsworth police tweeted: "Police are currently dealing with an incident in York Road #Wandsworth cordons are in place, Usk Road also closed - traffic disruption expected, motorists please avoid area."
One Twitter user reported that she heard gunshots outside of her window.
Sanober Ahmed wrote: "Gun shots heard outside my window and apparently a mans been shot... roads been cordoned off and police sniffer dogs searching the roads." SHARE THIS STORY USING ANY OF THE BUTTON BELOW ⬇ PLACE YOUR TEXT ADVERT BELOW ⬇⬇⬇