Police officers found the 17-year-old boy with stab wounds near Sutton railway station at about 19:10 GMT on Wednesday. He was pronounced dead at the scene.
The Met said the railway station was closed and several other roads in the area were shut.
The force has started a murder inquiry and said no arrests had been made.
Det Ch Insp Mike Nolan, who is leading the Met’s investigation, said: “This afternoon as Londoners go about their day, a family is reeling from the shock of losing a young family member to violence in our city.”
“Our investigation, though in its early stages, is moving at a pace and while we work to establish the circumstances,” he added.
In a statement, the Met said the teenager was treated at the scene by London Ambulance Service paramedics, but “died as a result of his injuries”.
It said the boy’s next of kin had been informed and were “being supported by specialist officers”, with a post-mortem examination yet to be scheduled.
Both Thameslink and Southern posted on X, formerly Twitter, to say the railway station was expected to remain closed for the rest of Wednesday.
Customers are advised to use alternative routes and check travel information before setting out.
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