The incident occurred in the men’s toilets at Reflex, George Hudson Street, at about 1.30am on 23 December 2012.
A 41-year-old local man was left unconscious and bleeding from a head injury as a result of the attack. He received treatment at York District Hospital.
The offender was captured on CCTV leaving the toilets.
He is described as white, aged in his late 20s or early 30s, with a medium build and a closely shaven head. He was wearing a white t–shirt with a dark logo on the front.
PC Cheryl Hunter, of the York Safer Neighbourhood Team, said:
Extensive enquiries have been carried out to identify the suspect. By releasing the CCTV image, we hope that people will come forward with a name or details that could assist the ongoing investigation.
If you would prefer not to contact the police directly, information can be passed to Crimestoppers without giving your name.
If you can help, please contact North Yorkshire Police on 101, select option 2, and ask for Cheryl Hunter. Or email email@example.com