The 66-year-old victim was assaulted in The Clansman, John Finnie Street, at about 14:40 on Wednesday.
He was treated for facial injuries at Crosshouse Hospital and later released.
The suspect was described as being white, in his 30s, about 5ft 8in tall, of stocky build, with thinning, short, dark hair and facial stubble.
He was wearing a black hooded jacket with a white logo on the right back of his shoulder, a grey T-shirt, blue jeans and white and black trainers.
Det Con Gary White said: "This would appear to be a totally unprovoked attack which has left this man with a serious injury.
"I'd like to speak to anyone who was within the pub at the time the assault took place or anyone who recognises the description of the person responsible."
Police are attempting to trace a man who carried out a "totally unprovoked attack" on a pensioner in a pub in Kilmarnock, East Ayrshire.