Police were called to London Road, close to West Croydon Railway Station, at 18:41hrs on Thursday, 18 November to reports of a fight involving a number of people.
Officers attended but no suspects or victims were found.
Shortly after 19:00hrs, police attended a south London hospital after a 14-year-old boy self-presented with stab wounds.
Despite the efforts of doctors at the hospital, the boy died a short time later. His next of kin have been informed and are being supported by specialist officers.
Formal identification has not taken place. A post-mortem examination will be carried out in due course.
Homicide detectives from the Met’s Specialist Crime Command are investigating.
Detectives would like to speak with anyone who was in the area of London Road and witnessed the fight.
There have been no arrests and enquiries continue.
Superintendent Andy Brittain, from the South Area Command Unit which covers Croydon, said: “This is a tragic loss of another young life to knife crime. Our thoughts are with his family and friends.
“We have worked throughout the night with our colleagues from Specialist Crime to establish what happened yesterday evening, and this investigation remains ongoing.
“I would urge anyone who has information regarding a fight that was seen close to West Croydon Railway Station at about 6.45pm to please come forward and speak with us. We need to identify who was there and what led to a 14-year-old boy being killed.
“Tackling violence remains our top priority but we need your help. A family is grieving; any piece of information you have could be vital in giving them the answers that they so desperately need.
“Local residents can expect to see additional officers in the area and a crime scene is still in place. If you have any concerns, please speak with officers or contact your local neighbourhoods team.”
Any witnesses, or anyone with any information is asked to call police on 101, quoting reference CAD 6179/18Nov, tweet @MetCC or call Crimestoppers, 100 per cent anonymously, on 0800 555 111.
+ Any young people who have information about violence, firearms or knife crime, can visit Fearless where they can pass on information anonymously – your I.P address will not be traced. Fearless is part of the Crimestoppers charity, and is also independent of the police.