The 18-year-old was walking his dog through Coronation Park, off Tickhill Road, Maltby, when he was approached by two men before one of them struck him in his face.
He was rendered unconscious in the attack, which happened between 1.30am and 2am on Saturday, October 28.
South Yorkshire Police said the man suffered serious facial injuries which required hospital treatment.
Both men were in their early 20s and one was wearing a hi-vis jacket.
Anyone with information should call South Yorkshire Police on 101 quoting incident number 179 of October 28.