Cops rushed to Friar Mews in West Norwood at 4.11pm to reports of a young man in a critical condition.
The victim – believed to be in his late teens – was pronounced dead at approximately 4.45pm.
Police have yet to formally identify the body and an investigation has begun.
The two men have been arrested on suspicion of murder and taken to south London police stations for questioning.
The Metropolitan Police said in a statement: “The Homicide and Major Crime Command have been informed and will be leading on this investigation.
“A number of enquiries including review of local CCTV footage and forensic analysis at the scene are in hand.
“Detectives are urging witnesses to speak to police at the scene, call officers, or Crimestoppers anonymously.
“Any witnesses and those with information should call police on 101/CAD 5335 or Crimestoppers anonymously on 0800 555 111.
“Young people can also pass on information in complete confidence via www.fearless.org.”