The assault took place at the corner of Hanover Street and Thistle Street at about 1:40am on Sunday, April 21st.
In a statement, police said the suspect is believed to have approached the victim from a premises on Hanover Street before carrying out the attack and making off in the direction of Queen Street.
He is described as a white man, aged in his 20s, around 6ft tall and of a broad, muscular build. He had short dark coloured hair and was wearing a grey or green coloured jumper at the time.
The injured man was assisted by a friend and was taken to the Edinburgh Royal Infirmary before being transferred to the Western General Hospital, where he is still receiving treatment.
Detective Constable Andy Cory, from Gayfield CID, said: “The victim was standing on the pavement on the corner of Hanover and Thistle Street with a friend when he was subjected to an unprovoked attack.
“As part of our inquiries we’re eager to hear from anyone who may have witnessed the assault, or who saw the suspect immediately prior to, or after, the attack. Hanover Street was busy at the time and we’d urge anyone who saw what happened to come forward.
“Anyone with any information to help us identify and trace the suspect is asked to come forward as soon as possible.”
Those with information should contact Police Scotland on 101, quoting incident number 0709 of 21 April, or Crimestoppers on 0800 555 111.
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