Police were called at around 5.40pm on Saturday, 6 February, to a stabbing incident in Willesden Lane, NW6.
Officers attended along with London Ambulance Service and found two males suffering stab injuries.
One of the males – aged in his 20s – was pronounced dead at the scene. His next of kin have been informed.
A second male – believed to be aged in his teens – was taken to hospital. His condition has been assessed as critical.
There has been no arrest at this early stage. An urgent investigation is underway to establish the full circumstances.
Anyone with information is asked to call 101 quoting CAD 5580/06feb. To remain anonymous, please call Crimestoppers on 0800 555 111.
A Section 60 is in place until 0930hrs on 7 February covering Brent and the north of Westminster.
Young people who have information about violence or knife crime can visit fearless.org where they can pass on information anonymously. Fearless is part of the Crimestoppers charity, and is also independent of the police. If you need help or information to support someone you suspect is involved in knife crime, or you want assistance yourself, then you can visit www.knifefree.co.uk or LondonNeedsYouAlive