The incident happened in Victoria Road near Queens Walk at around 4.55pm today (March 6).
Officers and London Ambulance Service attended and found a male, with no further details, who had been stabbed.
He was taken to hospital for treatment where his condition is currently unknown.
No arrests have been made as police enquiries continue.
A spokesperson for Metropolitan Police said: “Police were called at approximately 16:55hrs on Thursday, 6 March to reports of a stabbing in Victoria Road, Ruislip.
“Officers, London Ambulance Service and London’s Air Ambulance attended the scene and located a male – no further details – suffering a stab injury.
“He has been taken to hospital for treatment – condition awaits.
“A crime scene remains in place at the location and road closures are in place.
“No arrests; enquiries continue.
“Anyone with information that could assist police is asked to call 101 or ‘X’ @MetCC and quote CAD 5631/6Mar.
“You can also provide information anonymously to the independent charity Crimestoppers on 0800 555 111.”
