South Yorkshire Police said it had received 61 reports of incidents involving clowns since 21 September.
Officers are also investigating two reports of clowns armed with knives. In an interview, Owen Russell, 17, said he was walking home with friends, and bumped into the clown after taking a shortcut down an alleyway.
He said the clown started making sinister noises, before throwing a log at him, leaving him needing six stitches to a cut on his head.
Supt Colin McFarlane said: “There is a clear distinction to be drawn between young people who might be dressing up as we move into the Guy Fawkes and Halloween period, and those that are dressing as clowns in order to commit public order offences or harassment.”
The attack comes after dozens of recent reports across the UK of pranksters dressing up as clowns to scare members of the public.
The craze began in the United States last month, with reports of incidents in the UK emerging soon after.
Other reports to South Yorkshire Police include people dressed as clowns knocking on windows and doors.
Elsewhere, a 13-year-old girl was threatened with rape and having her throat cut, by a so-called clown prankster, Cumbria Police said.
In London, a teenager was arrested after reports of a man dressed as a clownscaring people at a university campus.