Police said the 30-year-old was waiting outside a takeaway in Glasgow's West End when he heard two men having a heated argument.
He intervened to try and calm them down but one of the men slashed him across the face before running off.
He was taken to the city's Queen Elizabeth University Hospital for treatment where medical staff describe his condition as stable.
The attack took place on St George's Road, near to Claredon Place at around 9.50pm on Sunday.
Detective Constable Ashley Convery said: "The man sustained a significant facial injury which will leave a permanent scar.
"From what we know at the moment, the attack seems totally unprovoked and only in response to the injured man trying to calm down an argument.
"Officers have been trawling CCTV to try and identify the man responsible for this attack and we have been in the area speaking to witnesses, however, we are keen to hear from the three people who helped the injured man after the attack.
"They left before police were called. Their information could prove vital in our investigation.
"This area can be quite busy and from CCTV we can see a number of people and cars in the area at the time of the attack.
"If you were about there at that time, please get in touch with police whether you think so or not, you may have seen or captured something on your dash-cams that will help us find the man responsible.
"The suspect is white, in his 30s, between 5ft 8in and 5ft 10 in tall and of stocky build. He has light brown hair which is short at the side and longer on top and was wearing a white jumper and blue jeans. He spoke with a Glasgow accent.
"The man he was having the argument with is also white and in his 30s. He is 6ft tall, slim, with dark hair, facial hair and had a Glasgow accent."