Police were called shortly after 10pm on Saturday, 31 October to reports of a group of young males fighting in Fernhead Road, W9.
Officers attended along with the London Ambulance Service.
Despite the efforts of emergency services, who provided first aid, a 19-year-old man was pronounced dead at the scene.
His next of kin has been informed and are being supported by specialist officers. Formal identification awaits.
A 20-year-old man was arrested on suspicion of murder at the scene and remains in custody at a central London police station.
Officers from the Met’s Specialist Crime Command (Homicide) are leading the investigation.
Anyone who saw what happened or has information about the incident is asked to call police on 101 quoting reference 9734/31Oct.
Alternatively, tweet @MetCC or contact Crimestoppers anonymously on 0800 555 111.