It happened outside the Randy Paddle pub, at the North Plantation, between 22:35 and 22:45 on 21 October.
Guernsey Police have described the incident as “an altercation” between two men.
Officers said the pair were both involved with one man punched in the face and knocked unconscious during the fight.
Guernsey Police want to hear from any witness to the incident.
“There were a number of patrons present in the premises at the time, as well as members of the public walking past. If you have any information regarding this incident you are asked to contact PC 159 Bourgaize on 222 222 or to contact Crimestoppers anonymously on 0800 555 111.”