It occurred at 11pm on Sunday 11 August 2013, as the victim, a 49-year-old man from York, walked along Hope Street.
As he passed a group of six to eight youths at the other side of the street, one them crossed the road and hit him in the face.
After the assault the offender stood over his victim with clenched fists, before heading off towards Piccadilly.
He is described as white, aged 17 to 20, with a roundish face.
As a result of the assault the man suffered a fractured cheekbone and eye socket, which required him to be treated as an in-patient at York District Hospital.
Detective Constable Chris Hogg, of York CID, said:
This was a nasty and unprovoked assault, which left the victim with severe injuries that will require further treatment.
If you were in the area just prior to the assault taking place and saw a group of youths, I ask you to come forward as soon as possible as you may have vital information that could progress my enquiries.
I am particularly keen to speak to other members of the group, who were not involved in this act of pointless violence. I urge them to do the right thing and contact the police straight away.
Anyone who can assist to identify the man responsible for this assault is asked to contact York CID on 101, select option 2 and ask for Chris Hogg. Alternatively information can be emailed to firstname.lastname@example.org or email@example.com.
Or call Crimestoppers anonymously on 0800 555 111 quoting reference number 12130138259.