At around 2.40 a.m. on July 10, a group of women reported being followed by a suspect in West Street and then in Duke Street.
The man assaulted one of the women.
The suspect was described as a white man in his forties with blond or light brown hair who was wearing a red jacket or jumper.
Officers have released a photo of a man they want to speak with about the incident.
Witnesses and anyone with information that could aid the investigation are asked to contact Sussex Police online or by phone at 101, quoting serial 412 of 13/07.