Experts say global warming has helped mosquitos carrying the potentially deadly disease to multiply.
They warn the virus could spread to Europe if the continent gets hotter.
In the first six months of 2019 Brazil recorded nearly 1.2 million cases of dengue fever and 388 deaths.
This compares to 175,000 cases and 220 fatalities in the first half of 2018, the Financial Times reports.
Scientists in South America’s largest country are particularly worried as the disease has carried on spreading during their winter, when infections normally decline.
Professor Ester Sabino of the University of São Paulo’s Institute of Tropical Medicine said global warming means mosquitos have had more hot days to multiply.
He added: ‘With global warming, Europe, for example, will also be exposed to these mosquitos and, consequently, diseases such as dengue.’
In recent years Brazil has suffered a series of outbreaks of mosquito-borne diseases such as the Zika virus and yellow fever.
Poorer people in Brazil’s favelas are more susceptible to dengue fever as the open water tanks they use to collect rainfall attract mosquitos.
The Brazilian Panel on Climate Change says average temperatures in the country are expected to soar by up to 6C by the end of the century.
But the country’s president Jair Bolsonaro is a known climate change sceptic who once said he would pull the country out of the Paris climate accord.
Experts have also pointed to heavier rainfall and a new strain of the disease from the Caribbean as factors behind the outbreak.
Others say the Zika virus has made Brazilians more susceptible to dengue fever.
The news comes after Thailand declared 2019 to be the worst year for dengue fever cases in half a decade.
By the middle of June – 28,785 people in the country had caught the disease and, 43 died – compared to 14,900 infections and 19 deaths the year before.