A 17-year-old boy was "slashed with a sharp object" in an Northolt cul-de-sac on Thursday night (August 2).
Metropolitan Police was called the the attack near Islip Manor Youth and Community Centre at 12am on Friday morning.
A teenager was found with "two slash injuries" at the scene and taken to a central London hospital from which he was later discharged.
A Metropolitan Police spokesman said: "Police were called at approximately 12am on Friday August 3 to reports of an assault in Palmer Close, Northolt.
"Officers and London Ambulance Service attended and found a male, believed aged 17, suffering two slash injuries. He was taken to a central London hospital and was discharged following treatment."
No arrests have been made in connection with the incident and police continue to investigate what happened.