Thousands of residents visited the Parana River to escape from the holiday heat, with temperatures reaching more than 100 degrees. But a school of the toothy fish spoiled the fun, attacking and inuring more than 70 people.
Officials said the palometa fish, a type of piranha, attacked the swimmers.
Though no-one was killed, a 7-year-old girl reportedly lost part of her pinky finger.
Rosario's director of lifeguards said it was an "isolated" incident.
"This is not normal. It's normal for there to be an isolated bite or injury, but the magnitude in this case was great... this is an exceptional event," he said.