The assault took place at around 3.45am on Saturday, 6th July outside the Diamond Kebab shop in Regent Street.
The victim was assaulted after intervening with a couple who were having an argument. He was punched in the face which left him with injuries to his jaw and teeth.
An Avon & Somerset police spokesman said: “An investigation has been launched and a man, aged 18, has been arrested on suspicion of assault. He is currently on bail while we continue our inquiries. As part of investigation we would like to hear from witnesses to the incident.”
Anyone who witnessed the incident or has other information which could help is asked to call PC Charlotte Roberts on the non-emergency 101 number.