This study was designed to evaluate the effects of graduated levels of nano-selenium (Nano-Se) in the fish diet at 0, 0.5, and 1mg/ kg on growth performance, feed utilization, organ indices, hematological, serum biochemical, and muscular chemical composition, and flesh quality parameters in the adult Nile tilapia, Oreochromis niloticus (female and male) for 10 weeks. Two hundred and forty adult Nile tilapia, male (n = 120) and female (n = 120), with an average initial body weight of 94.43±3.77 g and 103.22±2.77 (mean ± SD), respectively were subject to study. In adult males, a significant increase was recorded in final weight with decreases in red blood cells, hemoglobin, hematocrit, AST, total protein, albumin, and globulin, compared to females. The addition of Nano-Se resulted in significant reductions in all male and female growth performance parameters, while the feed conversion ratio increased. When compared to other levels, the dietary addition of 1mg of Nano-Se/ kg resulted in a significant increase in hemoglobin and white blood cells. Dry matter and fat content significantly increased as Nano-Se levels increased, while protein content significantly decreased. In terms of growth performance, feed efficiency, body indices, hematological, serum liver function enzymes, and flesh quality parameters, no significant interaction was detected between sex and different levels of Nano-Se. Finally, the results showed that the addition of Nano-Se had a negative impact on Nile tilapia growth performance, feed utilization, serum liver function enzymes, and flesh quality measurements. Thus, it was deduced that the tested levels of Nano-Se may be higher than the appropriate concentrations for both adult males and females of the Nile tilapia.