A 22-year-oold man was left with serious injuries, police have said, after being assaulted by a group of people near Avenue de Chartres.
The incident took place before 2.30am on New Year's Day.
The group are said to have made off towards Chichester railway station.
A spokesman for Sussex Police said: "Anybody who witnessed the incident, or has any information that could help with the investigation, is asked to contact police online or by calling 101, quoting serial 812 of 01/01."