This work was conducted to evaluate the effect of dietary organic selenium (OR-Se) supplementation on growth performance, carcass traits, nutrient digestibility, blood metabolites, and antioxidant capacity of broiler chicks that reared in a high temperature environment. A total number of 320 one-day old unsexed chicks (Cobb-500) were raised during summer season under temperature ranged between 29.9 to 33.3 ^°C. Birds were randomly assigned to four treatment groups of 80 chicks each, with four replicates each of 20 chicks, kept in floor pens. The 1^st group (T1) was fed basal diet, while the 2^nd, 3^rd and 4^th groups (T₂, T₃, and T₄) were fed diets supplemented with 0.3, 0.6 and 0.9 mg OR-Se/kg diet, respectively. The obtained results indicated that, groups supplemented with 0.6 and 0.9 mg OR-Se (T₃ and T₄) significantly improved growth parameters (body weight, body weight gain, feed conversion ratio), and nutrients digestibility. Also, these groups had higher (P< 0.05) carcass weight and lower (P< 0.05) abdominal fat. Besides, blood parameters including red blood cells, haemoglobin and haematocrit concentrations of chicks in these groups were clearly increased in comparison with chicks (T₂) or the control group (T₁). Furthermore, adding OR-Se to broiler diets statistically enhanced lipid profile, liver enzymes, kidney function, and antioxidant activity under high temperature environment. It could be concluded that dietary supplementation with OR-Se (0.6 and 0.9 mg/kg diet) had a positive effects on productive performance of broilers by improving growth performance, digestibility of nutrients, blood metabolites, and antioxidant capacity of broiler chickens practically under summer conditions compared to the others in the groups.