Emergency services rushed to the scene, after the stabbing was reported to the police at about 2.20pm off Tulse Hill, near Brockwell Park.
London Air Ambulance took the teenager to a major trauma centre. His condition is not yet known.
A London Ambulance Service spokesperson said: “We were called at 2.21pm today to reports of a stabbing off Tulse Hill.
“We dispatched a number of resources including two ambulance crews, a medic in a fast car, an advanced paramedic and an incident response officer with the first of our medics arriving within two minutes. We also dispatched London’s Air ambulance.
“We treated a teenager at the scene and took them to a major trauma centre.”
One local posted a video on social media showing the Air Ambulance landing at the scene.
The video shows the helicopter landing in Brockwell Park, near Brixton.
The user said several police cars were also in attendance.
More to follow…