The attack happened at around 11pm on Saturday July 8 outside the Yates's bar on Silver Street in Halifax.
The 33-year-old victim was punched and fell to the floor. He was treated in hospital for serious injuries.
The suspect is described as a stockily built white man white male in his late thirties . He was wearing blue jeans and a cream coloured top.
A West Yorkshire Police spokesman said: "Officers are appealing for anyone who can identify the male, who witnessed the incident or who has any information about the assault to come forward. Enquiries are continuing.
"Information can be passed to police by calling Calderdale CID via 101 quoting 13170312488. Information can also be passed anonymously to Crimestoppers on 0800 555 111."