The incident happened in Wealdstone High Street at around 10.30pm on January 18.
Officers and London Ambulance Service crews attended and found a man with injuries to his hand.
He was taken to hospital for treatment where his condition has been assessed as stable.
No arrests have been made as police enquiries into the circumstances continue.
A spokesperson for Metropolitan Police said: “Police were called at around 10.30pm on Saturday, 18 January to Wealdstone High Street to reports of a stabbing.
“Officers attended and discovered a man with injuries to his hand.
“He was taken to hospital in a stable condition.
“There has been no arrest and enquiries continue.
“Anyone with information is asked to call 101 quoting ref: CAD 7070/18Jan.
“To remain anonymous call Crimestoppers on 0800 555 111.”
