Emergency services rushed to Dudden Hill Lane, in Dollis Hill, shortly after 10.30pm on Saturday (May 30) after reports of a stabbing.
Once at the scene, police discovered a 15-year-old with stab wounds.
The boy was treated at the scene for his injuries, before being taken to hospital by the London Ambulance Service, where he was deemed to be in a critical condition.
No arrests have been made at this stage, but officers are investigating the incident “at pace”, the Metropolitan Police said.
A Met Police spokesperson said: “At 10.36pm on Saturday, 30 May, officers were called to reports of a stabbing on Dudden Hill Lane, Dollis Hill.
“Officers attended alongside the London Ambulance Service.
“A 15-year-old boy was treated at the scene for stab wounds and taken to hospital.
“His injuries have been assessed as life-threatening.
“Officers are continuing their enquiries at pace. No arrests have been made at this stage. A large cordon remains in place.”
Police are urging anyone with information about this incident to contact them on 101, quoting CAD 9258/30MAY, or contact Crimestoppers, anonymously, on 0800 555 111.
Dudden Hill Lane remains closed at the time of writing, with a crime scene in place between its junction with Villiers Road and its junction with Burnley Road, and a further cordon set up outside community centre SWAGAT Hall.
As well as police tape, officers, police cars and evidence markers have also been pictured at the scene.
Bus routes 226 and 302 are on diversion.
