Officials in the city of Caceres in Mato Grosso state say this is the first time they have had a problem with piranhas at the Daveron beach on the Paraguay river, where the aggressive fish began schooling about two weeks ago.
Elson de Campos Pinto, 22, was bitten on Sunday.
"I took a dip in the river and when I stood up, I felt pain in my foot," Pinto told G1.
"I saw that I had lost the tip of my toe. I took off running out of the river, afraid that I would be further attacked because of the blood. I'm not going back in for a long time."
"People have got to be very careful. If they're bitten, they've got to get out of the water rapidly and not allow the blood to spread," firefighter Raul Castro de Oliveira told Globo TV's G1 website today.
City officials said the beach will remain open because it's an important draw for tourists in Brazil's Pantanal region, known for its ecotourism.
"Everyone knows there are piranhas in the region and have always taken the necessary precautions," he said.
"What is different this time is that they've appeared where they never appeared before."