The city, which is located in the mountains north of Rio de Janeiro, was hit by torrential rainfall.
Houses in hillside neighbourhoods were destroyed and cars swept away as floodwaters raced through the city's streets.
The city's mayor has declared a state of emergency.
Search and rescue teams are combing the mud for survivors. Videos shared on social media showed extensive damage and vehicles floating in the streets.
"I found a girl who was buried alive," Wendel Pio Lourenco, a 24-year-old resident, told the AFP news agency while heading to a local church in search of shelter.
"Everyone is saying it looks like a war zone."
President Jair Bolsonaro, who is on an official trip to Russia, has said he will organise immediate help for the victims.
More than 180 soldiers have been deployed and more specialised search teams are expected to arrive later on Wednesday.