The downpours, which began on Friday, killed at least 11 people on Sunday when earth smothered homes in several towns in the state’s interior, authorities said.
According to the municipal government, three people from the same family died when a landslide destroyed their house in the town of Embu das Artes, while four other people were rescued by firefighters.
Four children died in Francisco Morato, Sao Paulo state governor Joao Doria said. Four other people died at Franco da Rocha, authorities said.
Deaths were also reported in Ribeirao Preto and Jau. Three of those killed were washed away by floodwaters.
“Since last Friday, the unrest caused by bad weather has claimed 18 lives, including seven children,” said the government of Sao Paulo state, citing civil protection.
Overflowing rivers forced 500,000 families from their homes, the state government said. Several roads and highways were also blocked.
Doria surveyed the damage by air on Sunday before announcing it was releasing 15 million reals ($2.7 million) to help the 10 worst-hit cities and 645 municipalities in Brazil’s most populous state, home to 46 million people.
Doria expressed her “solidarity” with the families of the 18 victims.
Brazil has been punished by heavy rains since the start of the year, with 19 deaths recorded earlier this month in eastern Minas Gerais, which borders Sao Paulo to the northeast.
In December, at least 21 people died and 358 others were injured during days of rain in Bahia state, which borders Minas Geras to the north. The worst of these floods came after two dams burst on December 24.
The floods have affected the country’s coronavirus vaccination campaign, with the city of Sao Paulo canceling planned vaccinations.
The heavy rainfall coincides with La Nina, a phenomenon that typically occurs every three to five years and leads to cooler than normal Pacific Ocean temperatures and affects weather across the globe.