Officers last night released a CCTV picture in an effort to identify a man after the offence in Albion Place at about 2.30am on Sunday, August 5.
The 18-year-old victim and a female friend were on a night out in Leeds city centre when they were followed by a group of four men who were asking the friend for a kiss.
When she refused the suspect grabbed her, officers said in their appeal.
Her friend intervened and was punched to the ground, leaving her with a severely bruised and swollen jaw.
She was treated at hospital after it was initially thought she had suffered a fractured cheekbone and eye socket.
The culprit was described as white, about 20 years old, wearing a red, white and black checked shirt.
Anyone who recognises him is asked to contact Detective Constable 3453 Tony Thompson at Leeds District CID via 101 quoting crime reference 13180389651 or call the independent charity Crimestoppers anonymously on 0800 555 111.