It happened at around 02:00 on Sunday 14 October, on the crossing between North Beach car park and the Liberation Monument.
The area would have been busy with revellers heading home from local night-clubs and bars and queuing for taxis at the nearby Weighbride rank.
Witnesses who may have seen the incident, which started off as an “altercation between two males in their early 20s” and led to one of them being punched, should contact PC 147 Le Page at Guernsey Police station on 725111.
Witnesses can also call Crimestoppers anonymously on 0800 555 111.
The victim was left with a broken tooth following the incident.
In a separate incident, Guernsey Police are also investigating an assault that happened about 02:20 on Sunday morning at the North Esplanade, near to the Tourist Information Centre.
That incident resulted in one man being taken to the Emergency Department at the Princess Elizabeth Hospital.
Any witnesses should call CID on 72511.
Pictured top: The area where the incident happened.