The incident happened in Brondesbury Villas near Kilburn High Street just before 5.15pm yesterday (October 7).
Officers and London Ambulance Service crews attended and found a boy injured.
He was taken to hospital where his injuries were assessed as not to be life-threatening or life-changing.
He was later arrested on suspicion of possession of an offensive weapon and remains in hospital at this stage.
A spokesperson for the Metropolitan Police said: “Enquiries continue.
“If you have any information that could help police please contact 101 quoting CAD5753/07OCT.”
A spokesperson for London Ambulance Service said: “We were called at 5.17pm on Tuesday 7 October to reports of a stabbing in Brondesbury Villas, NW6.
“We sent resources including ambulance crews, paramedics in response cars and an incident response officer.
“We treated a teenager at the scene and took him to a London major trauma centre.”
