Red Cross lifeguards told the swimmers to get out of the water at 5:30 p.m. Tuesday when a man at the Moll Grec beach said a blue shark had caused minor injuries to his arm, TheLocal.es said.
Five beaches in Catalonia were closed and warning flags were raised.
Authorities searched for the animal, but did not find it.
Doctors who examined the man's arm ruled out the possibility that the injuries had been caused by a shark, TheLocal.es said.
The incident was dismissed as a false alarm.