The 16-year-old’s condition is not life-threatening and another boy, 16, was found at 4.45pm in Brighton Road, Purley with head injuries, police said.
An eyewitness claimed they had seen a masked man fleeing the scene on Monday.
A bystander told MyLondon: “As I was walking through on my way back from work, I saw a masked man quickly left the scene.
“A schoolboy had been stabbed on the lower left side of his chest. Ambulances and an air ambulance arrived within 10 minutes to the scene”.
A Scotland Yard spokesman said: “Police were called to Brighton Road, Purley, on Monday to reports of a stabbing.
“Officers attended along with paramedics from London Ambulance Service (LAS).A 16-year-old boy was found with stab wounds.
“Another 16-year-old boy was found with a minor head injury.
“The boy with stab wounds has been taken to hospital. We await an update on his condition.”
There have been no arrests.