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Abstract Introduction Climate conditions may have influence on the transmission of COVID-19. Thus, this study aims to evaluate the impact of temperature and relative humidity on COVID-19 cases and deaths during the initial phase of the epidemic in Brazil. Methodology This is an ecological study based on secondary data. Daily data on new COVID-19 cases and deaths and on climate indicators was collected from February 20th to April 18th, 2020 (n=59 days), for all state capital cities in Brazil and the Federal District (Brasília). The studied climate indicators included mean temperature, temperature amplitude, mean relative humidity, relative humidity amplitude, and percentage of days with mean relative humidity less than or equal to 65%. Correlation and multiple linear regression analysis were performed for all cities and was also stratified by quintiles of the COVID-19 incidence rate. Results Mean daily temperature was positively correlated with the number of days until the first COVID-19 case was reported. A lower mean relative humidity was correlated with lower number of cases and deaths in Brazil, especially when the relative humidity was less than or equal to 65%. Higher temperatures and humidity amplitudes were correlated with lower COVID-19 mortality. Additionally, after controlling for humidity, cumulative cases of COVID-19 were inversely associated with temperature in cities with mean temperatures less than 25.8 °C. Conclusions Our findings indicate that variations in temperature and humidity across the Brazilian territory may have influenced the spread of the novel coronavirus during the initial phase of the epidemic in the country.