A teenage boy thought to be 15 years old.
The incident happened at 7.20pm, at New Park Road in Brixton Hill, Lambeth.
Scotland Yard has been rushed to hospital and police are on the scene.
The extent of his injuries are as yet unknown.
A statement from Lambeth MPS said: "A male, believed to be aged 15, was stabbed in #NewParkRoad #BrixtonHill #Lambeth around 19: 20hrs this evening.
Police are on scene No arrests at this early stage. "
Those with information are to call 101 with reference CAD 6232/26 Jan.