The victim, in his 20s, was taken to hospital with a gunshot injury in a private car around 2.30am on Saturday, say police.
Medics fought to save him, but he died around 3am.
Police say his family are being supported by specially trained officers.
Detectives believe he had been shot on the Westbury Estate, in St Rule Street, near Clapham Common.
A crime scene remained in place in the area on Saturday afternoon, as police appealed for witnesses to come forward.
No arrests had yet been made.
Detective Inspector Martin Thorpe, Specialist Crime, said: “The victim was a young man with his whole life ahead of him - a life that has been tragically cut short. My thoughts are with the victim’s family and friends at this difficult time.
“A young man has lost his life and his family desperately want answers.
“My officers and I are working to bring them the justice they deserve and your information could be a critical part of our investigation.
“If you were in the area around the time of this incident, or have doorbell, dashcam or CCTV footage that may have captured events, please get in touch with police at the earliest opportunity. If you want to remain anonymous you can share what you know via the independent charity Crimestoppers.
“I don’t underestimate the shock that an incident like this will cause the local community and I want to thank them for the patience while we continue to carry out our work at the scene.
“Residents will see both specialist and local officers in and around the location in the coming days. If you have any concerns or information that you think could assist please do speak to them.”
Anyone with any information or footage is asked to call police on 101 and give reference 884/11May. Alternatively contact Crimestoppers anonymously on 0800 555 111.
