Alistair Balfour is believed to have been set upon after intervening in a row between a male customer and staff at the busy bar.
The television repair man, of East Kilbride, Lanarkshire, was rushed to the town's Hairmyres Hospital following the incident at the Montgomerie Arms but died a short time later.
It is believed he was punched and suffered fatal head injuries after falling from a stool on to a stone floor following the attack on Sunday at about 5.45pm.
It is thought the attacker was acting aggressively towards staff and Mr Balfour had attempted to defuse the situation.
The attacker ran out of the pub after the altercation.
Police are studying CCTV footage from the scene and have appealed for people who were in the bar at the time to contact officers.
The pub remained closed yesterday and two police officers stood guard outside.
Mr Balfour was a well-known figure in the town, where he had run a television and radio repair business for several years.
A family member at the home Mr Balfour shared with his wife Ann, 65, refused to comment.
However, a friend, Elizabeth Denholm, said: "This is heartbreaking. He was a true gentleman and a lovely man. I was lucky to call him a friend."
Detective Inspector Mark Henderson, of Police Scotland's Major Investigation Team West, said: "We are following a number of lines of enquiry. A post mortem examination will take place to establish the exact cause of death, but we are treating it as murder.
"I would urge anyone who was in the Montgomerie Arms at the time who has not yet spoken to police to contact the inquiry team."