The incident happened in Beulah Hill at the junction with Spurgeon Road at around 5.30pm yesterday (January 22).
Officers were called to the scene and found a boy injured.
He was taken to hospital for treatment where his injuries were assessed as non life threatening.
No arrests have been made following the incident.
A spokesperson for Metropolitan Police said: “Police are in Beulah Hill, junction with Spurgeon Road, after calls around 17:30hrs on January 22 to reports of a stabbing.
“A 13-year-old boy was treated at the scene before being taken to hospital; his injuries are not being treated as life-threatening or life-changing.
“At this early stage there have been no arrests.
“Police are working to establish what happened.
“Anyone who can help police is asked to call 101 ref CAD 5240/22 Jan.”