Police said that the incident took place at about midnight today (October 28) next to a kiosk outside Burnt Oak station in Watling Road.
Officers were alerted by hospital staff after the man was brought into a central London hospital for treatment.
He currently remains there in a critical condition and officers are working to establish his identity.
Police believe the victim was punched by another man outside the station, before falling to the ground.
Detective Sergeant Emma Thompson said: “We know from CCTV that there were people in the area outside Burnt Oak tube station at around midnight who saw what happened and I would urge anyone with information to come forward.
“A man is lying critically ill in hospital as a result of this act of violence and we need to find who is responsible.”
Anyone with information is asked to contact police on 101 or ‘X’ @MetCC and quote reference CAD1419/28Oct.
You can also provide information anonymously to the independent charity Crimestoppers on 0800 555 111.