Police were called at 3.44pm on Friday to reports of an assault on Hulse Avenue in Barking.
A Met Police spokesman said: “Officers attended and found a boy, believed to be aged in his mid-teens, suffering from stab injuries.
“He was taken by the London Ambulance Service to an east London hospital for treatment. We await an update on his condition.”
Video on social media showed an air ambulance landing in a nearby park and police officers cordoning off a road while an ambulance worker treats someone lying in the street.
The police spokesman said there had been no arrests and enquiries continue.