The victim, believed to be 18, was rushed to hospital just before 5pm after he was treated in Grove Park Road in Lewisham.
Scotland Yard said detectives from the Trident and Gang Crime Command have been informed as police investigate the attack.
A Met spokesman said: “Police were called by the London Ambulance Service at 4:55pm on Monday to reports of a male suffering from stab injuries on Grove Park Road in Lewisham.
“Officers and London's Air Ambulance attended.
“The victim, believed to be aged 18, was taken to a south London hospital where he remains in a critical condition.
“Detectives from the Trident and Gang Crime Command have been informed. There have been no arrests and enquiries continue.”
Pictures taken from the scene show an air ambulance landed on a nearby football pitch while neighbours said roads had been taped off.
One woman tweeted: “Get home to everywhere closed off cause someones been stabbed... lovely.”
