The objective of this study was to compare the effects of synthesized nano-selenium (NS) and commercial inorganic selenium (Se) on immunity, behaviour, and performance of Arbor (AB) and Ross (RB) broilers that were exposed to heat stress of 40 °C for 6 – 8 hours daily over 38 days. Two hundred and ten one-day-old broilers of two breeds were supplemented with 0.5 mL/L of NS or Se in their drinking water. Two hundred sera, 200 intestinal swabs, and 1000 internal organ and tissue samples were collected. Weight gain, performance index, behavioral indices, total antioxidant capacity, malondialdehyde, superoxide dismutase, immunoglobulin G, immunoglobulin M, serum total protein, albumin, alanine aminotransferase, aspartate aminotransferase, and serum creatinine concentrations increased (P <0.01) in RB compared with AB when supplemented with NS. Meanwhile, NS supplementation decreased (P <0.01) water intake and the logarithmic bacterial counts of the intestine and breast in RB and AB, respectively. Histopathology revealed mild leukocytic infiltration and mild vacuolar degeneration in hepatocytes, and focal leukocytic infiltration, mild congestion, and cytoplasmic vacuolation in the myocardium of RB. Photomicrographs showed a mild lymphoid depletion in the spleen, while histopathology of the bursa of Fabricius revealed a normal follicular epithelium and normal lymphoid follicles with mild inter-follicular fibrosis in RB that were supplied with NS as opposed to AB, which expressed more severe pathological affections from heat stress. Thus, NS was more effective than Se in allowing broilers to respond to heat stress.