Abstract: Background and Objective: Broiler reared above the condition of thermoneutral zone will be vulnerable to environmental heat stress and exhibit behavioural and physiological changes. This study is to evaluate the effects of supplementation of potassium chloride (KCl) in drinking water on broiler performance and physiological responses under conditions of environmental heat stress. Methodology: A total of 200 days old broiler chicks were evaluated in this study. They were randomly divided into 20 plots and reared for 5 weeks. A completely randomized design with 5 treatments and 4 replications were assigned in this study. The treatments were symbolized as R0, R1, R2, R3 and R4, which is respectively supplemented with KCl as much as 0, 0.25, 0.50, 0.75 and 1.00% (w/v) in drinking water. Variables observed were feed and water intake, body weight gain, feed conversion ratio, the amount of potassium, sodium and chloride absorbed, mortality, the panting percentage, cloacal temperature, numbers of leukocytes and blood pH. Results: The results indicated that the supplementation of KCl in drinking water did not significantly (p>0.05) affect the panting percentage, however, significantly (p<0.05) affected the feed and water intake, body weight gain, feed conversion ratio, the amount of potassium, sodium and chloride absorbed, cloacal temperature, numbers of leukocytes and blood pH. Conclusion: It was concluded that the supplementation of 0.50% KCl in drinking water was the optimal level for improving performance and physiological response of broiler chickens under environmental heat stress condition.