UOL Noticias reports that the attacks occurred at beaches near a dam just outside the town of Jose de Freitas, in the northeastern state of Piaui.
More scattered attacks have been reported there for the past two months.
The once-popular resort has now been abandoned, and officials say overfishing of species that the piranhas normally eat is the main reason the fish have turned to humans.
Efforts to have sport fishermen keep all caught piranhas while throwing back all other types of fish failed to curb the beach attacks.
"Since they have no predators, piranhas have started attacking people on the beach," said local environmental official Romildo Mafra.
The Brazilian Institute of Environment and Renewable Natural Resources said it has released 100,000 tilapias into the reservoir’s food chain to help restore the ecological balance and make it safe for humans to once again go back into the water.