Abstract Environmental thermal comfort provides higher efficiency in the exploitation of dairy farming, as the responses of animals to the environment are decisive in the activity’s success. This study aimed to evaluate heat stress in dairy cows in the municipality of Humaitá, located in the south of theAmazonas State (AM), Brazil.Air temperature (TA) and relative humidity (RH) data were used to calculate the temperature humidity index (THI) in the dry (August 2020) and rainy seasons (January 2021). The August data were collected at the automatic weather station belonging to the National Institute of Meteorology. The January data were collected by a Hobo® U23-001 thermo-hygrometer installed at Fazenda Nossa Senhora Aparecida. The hourly and monthly means of the data were calculated and, subsequently, boxplots were plotted. TA and RH were 0.65 °C and 0.64% below the provisional climatological normal (PCN), respectively, for August, while January presented 0.77 °C and 3.61% above the PCN, respectively. THI values ranged from 56.25 (comfortable) to 84.68 (emergency) in August and 74.15 (comfortable) to 84.07 (emergency) in January. The maximum THI was observed in August, as well as the largest range. The results allow inferring that heat stress in dairy cows in the municipality of Humaitá-AM presents a state of alert even for cross-breed animals. Thus, the use of shading in pastures is suggested to minimize the effect of heat stress on the productive performance of the animals.