This study was conducted to investigate the effects of selenium nanoparticle (Se-NP) supplementation on the growth performance, carcass composition, antioxidant status, hepatic enzyme activities, and immunity of Cirrhinus mrigala. For this purpose, fish with an average initial weight of 7.44 ± 0.04 g were fed five experimental diets containing 0 (control), 0.25, 0.5, 1, and 2 mg kg-1 Se-NPs diets for 90 days. The analysed selenium (Se) contents of the diets were 0.35, 0.64, 0.92, 1.43, and 2.39 mg kg-1. Twenty five fish were randomly distributed in each of 5 aquarium (36 × 23.7 × 24.3 inches) in triplicate. The results showed that supplementation with Se up to 0.92 mg/kg significantly increased (p<0.05) weight gain, weight gain% (WG%), and specific growth rate (SGR) by 34%, 33%, and 16%, respectively, compared to the control diet. Dietary Se concentrations up to 0.92 mg/kg significantly increased the crude protein and crude fat and reduced (p<0.05) the moisture content as compared to the control group. Fish fed 0.92 mg kg-1 Se had significantly lower malondialdehyde (MDA) contents and higher activities of catalase, superoxide dismutase, and glutathione peroxidase in liver and serum as compared to other experimental diets. Moreover, a significant increase (p<0.05) in the level of serum immunoglobulin and lysozyme (LYZ) activity was recorded in fish fed 0.92 mg/kg Se diet. Moreover, the highest (p<0.05) values of aspartate transaminase (AST) and alanine transaminase (ALT) were recorded in fish fed 2.39 mg/kg Se level. However, serum alkaline phosphatase (ALP) activity remained unaffected by dietary treatment. Broken-line regression analysis indicated that 0.83 mg/kg Se is required for the optimum growth performance of C. mrigala.