Officers believe the man they want to speak to could hold vital information about the attack in a pub on Silver Street, Doncaster town centre, on Monday, May 13.
South Yorkshire Police said a 45-year-old man was talking at the bar when he was approached by a man and punched in his face at around 3am.
He was taken to hospital and treated for facial injuries.LOCAL: Emergency services deployed over concerns for welfare of man in river in SheffieldAnyone with information should call South Yorkshire Police on 101 and quote reference number 14/78976/19.Crimestoppers can also be contacted, anonymously, on 0800 555 111.