Sussex Police said he was found with serious injuries and a man has since been arrested on suspicion of grievous bodily harm.
Police were at the scene gathering evidence during the night and cordoned off part of the area.
A Sussex Police spokesperson said: “Officers attended Wellington Square at around 7.35pm on Tuesday following a report of a man found unconscious.
"He was taken to hospital for treatment to injuries described as serious but not life-threatening or life-changing.
"Enquiries established the man had been assaulted and a 30-year-old man has been arrested on suspicion of grievous bodily harm and remains in custody at this time.
"Anyone who saw what happened or who has any other information is asked to contact police online or by calling 101 quoting serial 1248 of 28/06.”