The incident happened in Prince Imperial Road at around 4.20pm yesterday (January 6).
Officers and London Ambulance Service crews attended and found a boy who had been stabbed.
He was treated at the scene before being taken to hospital where he remains in a critical condition.
No arrests have been made as police enquiries into the circumstances continue.
A spokesperson for Metropolitan Police said: “Police were called at approximately 4.20pm on Monday, 6 January, to reports of a stabbing on Prince Imperial Road, SE18.
“Officers responded to the scene and found an 18-year- old man suffering from stab wounds.
“He was treated at the location before being transported to the hospital, where he remains in critical condition.
“Investigations into the incident are ongoing, and no arrests have been made at this time.
“Anyone with information is asked to call 101, quoting reference CAD 4354.
“Alternatively, anonymous tips can be provided to Crimestoppers at 0800 555 111.”
