The press office of the Rio de Janeiro fire department announced the death toll Thursday.
Rio's Mayor Marcelo Crivella decreed three days of mourning for the victims.
Crivella said: "In two hours the amount of rain that fell on the city was equivalent to the rainfall for the entire month."
He added: "Meteorologists never imagined that the storm would be as violent as it was."
The Rio Operations Center said that strong winds of up to 68 mph (109 kph) knocked down trees and caused heavy flooding in several streets.
The center monitors weather, traffic, trash collection, electricity, water, gas and emergency situations, among other things.