The storms, which have caused the greatest devastation in the state in recent years, also left 60 people missing and 10,242 displaced in 154 cities, according to Rio Grande do Sul's civil defence.
"It's not just another critical case; it's the most critical that the state will probably have recorded in its history," state Governor Eduardo Leite said on Thursday in a live broadcast on social media, adding the situation is worse than 2023's rains in the state.
More than 300,000 people have also been left without electricity after a dam at a small hydroelectric power plant burst on Thursday, the state's main utilities company said.
President Luiz Inacio Lula da Silva flew over the affected areas and met with Governor Leite in Santa Maria on Thursday for an emergency meeting.
"I told the governor and my ministers that the federal government will make every effort. ... We will take care of this with great care and respect," Lula said during the meeting.
In a video posted on social media, Leite called for coordination in the efforts to rescue people, asking for "full force" as he declared a state of public calamity citing the risk faced by the state as a result of climate events.
Lula told Leite in a call late on Wednesday he would send as many men as necessary to help deal with the situation, the president's office said.
Brazil has faced a recent spate of natural disasters. More than 50 people were killed in Sao Paulo state last year after massive downpours caused landslides and flooding.
