The man was knocked down at the junction of Westburn Avenue and Westburn Middlefield in Edinburgh.
The incident happened at 12.40am on Saturday and police say that the 18-year-old was taken to the Royal Infirmary of Edinburgh to be treated for severe head injuries.
A Scottish Ambulance Service spokesman said: “We attended an incident from a 999 call on Westburn Avenue.
“The call was from someone who wasn’t on scene.
“An 18-year-old was hit by a taxi and was taken to hospital to be treated.”
Police are looking to speak to anyone who was in the area at the time and witnessed the incident and have asked them to contact 0131 311 3418 or 0131 311 3131.