The 16-year-old was found collapsed in Tulse Hill on Bonfire Night and police originally thought it was a shooting.
Scotland Yard said: "Despite the efforts of paramedics, the male was pronounced dead at the scene at 11.41pm. His next of kin are aware.
"A post-mortem examination held on Thursday at Greenwich Mortuary gave cause of death as a stab wound.
"Officers are also appealing to anyone who may have captured footage or images of the incident on their mobile phone to get in contact.
Murder Squads Detective Chief Inspector Richard Vandenbergh said: "I am appealing to anyone who has information that could assist this investigation to come forward and speak to police.
"We believe the victim was attacked by a number of suspects and the altercation would have undoubtedly drawn the attention of those who were in the vicinity.
"Did you see or hear anything? If so, no matter how insignificant you think it may be, please call – your information could prove to be crucial."
Chief Superintendent Simon Messinger, Commander of the Central South Command Unit, said: "We have seen a spate of fatal stabbings in south London recently with five murders over the course of the last week.
"One murder is one too many with the devastation it causes to families, friends and communities difficult to comprehend.
"This includes utilising our local resources but also supplementing them with resources from across the Met, including the Violent Crime Taskforce.
"Extra officers have been assigned to the Central South Command to work alongside local officers – you will see them out and about on patrol.
"And our prevention work, which has been key in helping to stem the overall tide of violent crime, remains ongoing.
"Across the Met this year, officers have conducted over 21,000 weapons sweeps and taken hundreds of weapons off the streets. This work will continue.
"I would urge anyone who has witnessed a crime, violent or otherwise, to contact police as soon as possible and help us to remove violent individuals from our streets.
"Any information, no matter how big or small, could be of huge help to our investigations and all information provided will be treated in the strictest confidence.
"If you have any questions or concerns, please do approach any officer that you see patrolling the area. They will be happy to speak with you."
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