High environmental temperatures lead to metabolic changes, body weight reduction, and high mortality in chickens, affecting poultry production worldwide. This study aimed to evaluate the effects of heat stress, assessed by the temperature-humidity index (THI), on the growth performance, physiological response, carcasses, and hematological traits of broilers. A total of 200 broilers (between 17 and 31 days old) were kept in thermoneutral conditions (21 °C
60% relative humidity, RH) for 3 days during the adaptation period, followed by 14 days of exposure to the experimental treatments. The broilers were randomly assigned to 4 groups of equal size and raised in a temperature-humidity controlled chamber with THI ranging from 67 (21 °C
RH 60%) to 84 (33 °C
RH 60%). In boilers subjected to severe heat stress (THI 84), feed intake (-30%) and body weight gain (-51%) were decreased (P <
0.05) in comparison with the performance parameters of broilers under thermoneutral conditions (THI 67). Moreover, under heat stress, the respiration rate and rectal temperature significantly increased (P <
0.05), whereas blood parameters showed reduced levels of red blood cells, hemoglobin, white blood cells, lymphocytes, and electrolytes (K+, Na+). Regarding carcass traits, differences were observed in broilers exposed to severe heat stress, particularly in the reduction of the bursa of Fabricius as an immune organ growth index (P <
0.05). This study shows that severe heat stress, as revealed by the high THI levels, alters the physiological reactions and metabolic processes of broiler chickens, leading to negative effects on their growth.