A Sussex Police spokesman said: “The victim was with a friend in Robertson Street at around 3.15am on Tuesday May 3 when they walked past a group of men.
“One of these group was reported to have made an inappropriate comment. The victim attempted to intervene in the confrontation that ensued and was punched once to the face.
“As well as losing consciousness, he also sustained a fractured jaw and underwent treatment in hospital.”
The attacker was described as a white man, of slim build with short dark hair. He was wearing a blue hooded top with a logo on the left breast, dark blue shorts with white writing down the left leg and dark trainers.
Anyone with CCTV of the incident or information about it, is asked to report online, or call 101, quoting serial 845 of 03/05.