The Met Police were called shortly before 4pm to Northwick Park in Harrow following reports of a stabbing.
Officers at the scene found a male, believed to be 17-year-old, suffering from stab injuries.
The victim was treated by officers before emergency crews from the London Ambulance Service arrived.
He was pronounced dead at 4.31pm. Next of kin have been informed.
There have been no arrests made.
This is a developing news story, more to follow.