Scotland Yard said the victim, believed to be in his late teens, was attacked in Neasden on Friday afternoon.
Emergency services were called to reports of a stabbing in Tanfield Avenue shortly before 4pm.
Medics fought to save him but he was pronounced dead at the scene.
A Met Police spokesman said: “A post-mortem examination and formal identification will be arranged in due course. We await confirmation that next of kin have been informed.”
Roads in the area have been cordoned off.
There have been no arrests.
Hours later at about 6pm, a 16-year-old boy was stabbed in Longheath Gardens, Addiscombe, in south-east London.
Police said he was taken to hospital with potentially life-threatening injuries. No arrests were made.
Anyone with information should call police on 101 or to remain anonymous call Crimestoppers on 0800 555 111.