Police were called shortly after 6pm on Tuesday, 12 September to reports of people fighting on North Walk in New Addington.
Officers attended along with the London Ambulance Service. Two men were found nearby with stab wounds.
One of the men, aged 20, died at the scene. His family has been informed.
A post-mortem examination will be held in due course.
The second man, aged 19, was taken to hospital for treatment. His injuries are not believed to be life-threatening.
A 20-year-old man has been arrested on suspicion of murder.
Officers from Specialist Crime are investigating.
Detective Inspector Jason Fitzpatrick, said: “This fatal attack happened in daylight and I’m sure it would have been witnessed by a number of people.
“We are carrying out a number of enquiries, including a review of local CCTV. I would ask that anyone who has a video doorbell, or who was driving in the area with a dash cam, to please check their footage.
“It is vital that we establish what happened yesterday evening and locate those responsible.”
Superintendent Mitch Carr, from the South Area BCU, said: “The local community will share our shock and concerns following this tragic loss of a young life.
“You will see additional police officers in the area and I would ask that you speak with them if you have any information. It is only by working together that we can reduce violent crime, and we need your help with this investigation.”
Any witnesses are asked to call 101 or post @MetCC quoting reference CAD 6879/12Sep. To remain 100% anonymous contact the independent charity Crimestoppers on 0800 555 111 or visit crimestoppers-uk.org.
