Officers were called to reports of a fight in Isleworth at about 10.35pm on Friday.
The boy, believed to be 17, was found outside a block of flats in Union Lane.
Paramedics battled to save him but he was pronounced dead at the scene a short time later.
There have been no arrests, Scotland Yard said.
A police spokesman added: “A post-mortem examination and formal identification will be arranged in due course. Officers have begun the process of informing next of kin.
“An urgent investigation is underway, led by detectives from the Homicide and Major Crime Command supported by colleagues from the West Area Command Unit.”
Any witnesses or anyone with information that may assist police is urged to call 101 quoting CAD 8396/mar22. To remain anonymous call Crimestoppers on 0800 555 111.