This study was conducted to examine the influence of dietary supplementation of biological nano-selenium (BNSe) on productive performance, hematology, blood chemistry, antioxidant status, immune response, cecal microbiota, and carcass traits of quails. In total, 180 Japanese quails (1 week old) were randomly allocated into four groups, with five replicates of nine chicks each in a complete randomized design. The 1st group was fed a control diet without BNSe, and the 2nd, 3rd, and 4th treatments were fed diets supplemented with BNSe (0.2, 0.4, and 0.6 g /kg feed, respectively). The best level of BNSe in body weight (BW) and body weight gain (BWG) parameters was 0.4 g/kg diet. Feed conversion was improved (P < 0.01) by adding BNSe in quail feed compared with the basal diet without any supplementation. The inclusion of different BNSe levels (0.2, 0.4, 0.6 g/kg) exhibited an insignificant influence on all carcass traits. The dietary addition of BNSe (0.4 and 0.6 g/kg) significantly augmented aspartate aminotransferase (AST) activity (P = 0.0127), total protein and globulin (P < 0.05), white blood cells (WBCs) (P = 0.031), and red blood cells (RBCs) (P = 0.0414) compared with the control. The dietary BNSe supplementation significantly improved lipid parameters, antioxidant and immunological indices, and increased selenium level in the blood (P < 0.05). BNSe significantly increased (P = 0.0003) lactic acid bacteria population number and lowered the total number of yeasts, molds, total bacterial count, E. coli, Coliform, Salmonella, and Enterobacter (P < 0.0001). In conclusion, adding BNSe up to 0.4 and 0.6 g/kg can boost the growth, lactic acid bacteria population number, hematology, immunological indices, antioxidant capacity, and lipid profile, as well as decline intestinal pathogens in growing quail.