A 33-year-old man was playing the fruit machine at the Rosevale Tavern in Partick around 6pm when he was approached by a man who was unknown to him.
An altercation took place and the man assaulted the victim before making off from a side entrance of the premises.
CCTV images show the suspect headed along Dumbarton Road in the direction of Glasgow City Centre.
The injured man was taken to the Queen Elizabeth University Hospital for treatment to a serious facial injury.
The suspect is described as in his mid 20s with brown hair which was longer at the top and shaved at the bottom. He was wearing a black quilted waist-length jacket, dark blue jeans and dark trainers.
Detective Constable Jen Adams from Partick CID said: “This was a completely unprovoked attack which will leave the victim scarred for life.
“Officers have been conducting extensive enquiries including examining CCTV and speaking to people who were within the pub at the time of the incident.
“Someone must know who did this and I would ask anyone with information to contact officers at Partick CID on 101.
“If you would prefer to provide your information anonymously you can call CRIMESTOPPERS on 0800 555 111.”