With more rain predicted Sunday, the deluge pounded the states of Rio de Janeiro and Espirito Santo, where authorities described a chaotic situation due to flooding.
The death toll rose there from four to 12 on Sunday as rescuers advanced.
The most affected municipality is Mimoso do Sul, a town of almost 25,000 inhabitants located in the south of Espirito Santo, where at least 10 people died in floods, though officials fear the toll may yet rise.
State Governor Renato Casagrande described the situation as "chaotic," saying that so far it has not been possible to assess the damage in some of the more isolated areas, with fears the toll could yet rise.
At least eight people have been killed in the neighboring state of Rio de Janeiro, officials said, most of them caused by landslides.
Four of the deaths in Rio state occurred when the storm caused a house to collapse in the city of Petropolis, 70 kilometers (45 miles) inland from the capital.
Search teams rescued a girl buried for more than 16 hours there. Her father, who was found dead next to her on Saturday, had "heroically protected the girl with his body," a neighbor told AFP.
The deluge came as Brazil, South America's largest country, suffers through a recent string of extreme weather events, which experts say are more likely to occur due to climate change.
Such environmental tragedies "are intensifying with climate change," President Luiz Inacio Lula da Silva said in a post on X, formerly Twitter, adding that thousands had been left homeless by the storm.
He expressed sympathy for the victims, and said his government was working with state and local authorities to "protect, prevent and repair flood damage."
Around 90 people have been rescued since Friday, according to a bulletin from an emergency committee comprising Rio government and civil defense officials.
Images on local media showed rivers of water, mud and debris rushing down slopes in picturesque Petropolis, which in February 2022 saw at least 241 deaths from another catastrophic storm.
In Mimoso do Sul, a fire truck was seen being dragged down a street by currents, while images released Saturday by the state fire department showed entire neighborhoods under water, with only the roofs of houses visible.
The National Institute of Meteorology had predicted a severe storm, particularly in Rio, with rainfall of 20 cm (7.9 inches) a day from Friday through Sunday. Normally, the area receives 14 cm (5.5 inches) of rain in all of March.
Rio authorities had declared an administrative holiday on Friday as the storm approached and urged people to stay home.
The storm follows a record heat wave, when humidity helped send the heat index soaring above 62 degrees Celsius (143 degrees Fahrenheit).
