The baby, apparently two months old, was rescued Saturday afternoon by a couple who heard her crying in the Pampulha Lagoon in Belo Horizonte, a city about 300 miles northeast of Sao Paulo.
Amateur footage broadcast on Brazilian TV showed rescuers using a long tree branch to pull the black plastic bag out of the water. They opened the bag and found the girl — wearing a pink dress — soaked inside.
"I heard what sounded like a cat," Jose da Cruz told Globo TV. "Then the noise was increasing and it caught my attention."
A small wooden board inside the bag apparently kept it from submerging.
"She is in good condition," Dr. Claudia Guimaraes said. "It's a miracle she survived the heat and somehow didn't drown."
Authorities had no leads on why the baby was thrown in the lagoon.
The Pampulha Lagoon is located in downtown Belo Horizonte, in one of the city's most famous parks, often crowded with weekend visitors.