Police were called to Uxbridge Road in Shepherd’s Bush at 9.20pm on Wednesday, 26 June following reports of a stabbing.
Officers, the London Ambulance Service and London’s Air Ambulance attended. An 18-year-old man was found suffering from a stab injury.
Despite the efforts of attending paramedics he died at the scene a short time later.
His next of kin have been informed. Formal identification awaits.
A post-mortem examination will be held at Uxbridge Mortuary on Friday, 28 June.
Detectives from the Homicide and Major Crime Command are appealing for any witnesses or anyone with information to come forward and speak with them.
Detective Chief Inspector Rob Pack said: “I would like to hear from local residents or anyone who was in the area at the time of the stabbing who is yet to come forward.
“We have already carried out extensive CCTV enquiries in the vicinity and I am now particularly keen to speak with anyone who saw two males, possibly aged in their teens, who we know ran from the scene heading in the direction of Wormholt Park.
“They then took the underpass under the Westway towards East Acton Underground Station. This is a busy area and a number of people would have been in the area at the time. Did you see them? They may well have stood out as they would have been out of breath.
“It is important that we speak with them and find out exactly what happened.”
There have been no arrests and enquiries continue.
Anyone with information is asked to contact police on 101 quoting CAD reference 8782/26June.