Police are appealing for witnesses after a flare was set off inside the stadium during the Swansea City v Manchester United football match.
The flare was let off 15 minutes into the game - which ended 1-1 - after Manchester United scored.
Police said it was discharged from the crowd in the Manchester United end of the ground before hitting the roof and ricocheting back down into the crowd in Liberty Stadium in Swansea.
Detectives said it was fortunate that the flare missed any spectators and that no-one was hurt following the incident.
Detective Inspector Jeff Handley, of South Wales Police, said: "We are proud of the fact that the Liberty Stadium promotes a family atmosphere and welcomes spectators from all over the UK.
"This was a very foolish act by an irresponsible individual. We are fortunate that nobody was seriously hurt today.
"It's important that anybody with information contacts us so that we can identify the individual and let him or her explain their actions to a court."
Detectives are appealing for anyone with information to contact South Wales Police 01656 655555 or 101.