Police in St Helens want to speak to teenagers who assaulted another youngsters outside McDonalds on Chalon Way.
One of the five-strong gang of attackers demanded money from the victim before pouring a drink on him.
Moments later the same attacker returned and punched the victim in the face.
He was taken to hospital for assessment for a split lip and a sore head.
Det Insp Louise Birchall said: “This was a nasty and completely unprovoked attack on a young person, who was simply spending time with a friend.
“We are keen to speak to the male pictured, who may have vital information to help our investigation.
“If you recognise this man or witnessed the attack itself, please come forward and we will act on all information given and ensure that St Helens remains a safe place to visit, live and work.”
Anyone with information is asked to contact the social media desk @MerPolCC, call 101 quoting reference 0518213556 or you can call or the independent charity Crimestoppers anonymously, on 0800 555 111.