Jersey Police say a young woman was sexually assaulted in Snow Hill car park on Tuesday 22 February.
The alleged incident happened between 11:15pm and midnight.
A 63-year-old man was arrested in the area a short time later and has been released on police bail whilst enquiries continue.
Officers want to speak to anyone who was in the car park at the time - particularly two men who were sitting in a car for between 15 and 20 minutes.
Anyone with information is asked to call the police on 612612 or Crimestoppers anonymously on 0800 555 111.