A seven-year-old boy was hit in the face by a flare at a football match between Barnsley and Hull City.
The flare was thrown into the crowd by a Hull fan at Barnsley's Oakwell stadium shortly before half-time on Saturday, South Yorkshire Police said.
The boy was treated for his injuries at the ground and was able to return to watch the rest of the match.
Officers are looking at CCTV from the stadium and a 17-year-old boy is being questioned by police.
Ch Insp Blake Neale from Humberside Police said: "Behaviour of this nature will not be tolerated as families should feel safe to be able to go to watch their teams play without the fear of this type of incident.