The 18-year-old suffered a serious head injury during an altercation with another man of the same age at about 1.15 am on Easter Monday in Liberation Square.
The alleged attacker was arrested at the scene on suspicion of grave and criminal assault and has been released on bail pending further inquiries.
Anyone with information – particularly two men who were seen talking to the two teenagers – is being asked to phone the States police on 612612 or Crimestoppers on 0800 555 111.
