The 37-year-old victim was at the corner of Leith Street and Princes Street when he became involved in an argument with another man at around 6.20pm on Thursday.
The suspect punched the victim, causing him to fall and strike his head on the street.
The victim was taken to Edinburgh Royal Infirmary.
Advertisement
Hide AdAdvertisement
Hide AdA police spokesman said: "We're appealing to any passers-by to come forward."