A gang of skateboarding youths confronted a man and woman before one of them punched the man in St Kilda early on Friday.
Senior Sergeant Craig Dinnissen said police were appealing for witnesses to the assault, which happened in Douglas St at 1.36am.
The man and woman were approached by six youths with skateboards, before the man was punched by one of them.
It was unclear what sparked the assault, and the youths were unknown to the pair, Snr Sgt Dinnissen said.
Inquiries were ongoing.
