An eight-week nutritional study was carried out to examine the effects of dietary selenium nanoparticles (Se-N) on performance in whiteleg shrimp (Penaeus vannamei) juveniles (1.55 ± 0.04 g, means ± standard deviation). A basal diet (∼ 38 % crude protein, 18.5 kJ/g crude energy) was supplemented with Se-N at five levels, including 0 (control), 0.2, 0.4, 0.8, and 1.2 mg/kg. Three hundred shrimps were distributed randomly into 15 circular fiberglass tanks containing 150-L seawater (20 shrimp/tank). Each dietary treatment was offered to shrimps in three tanks. Shrimps were handfed three times daily based on 7 % of their initial biomass. Water temperature and salinity were 30 ± 0.4 °C and 40 g/L, respectively. The hepatopancreas (HP), hemolymph, and gut were sampled after 56 d of the feeding trial. Shrimps fed Se-N supplemented diets had positive quadratic trends in growth performance (P = 0.011) and feed conversion ratio (P = 0.006). Selenium level increased in the shrimp’s whole body with increasing Se-N in diet and showed both linear and quadratic treands. The activities of plasma phenoloxidase (PO) and, catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx) in the HP were increased by supplementing diet with Se-N and showed positive quadratic treands. The malondialdehyde (MDA) level significantly decreased in the HP of shrimps fed Se-N and had a quadratic treand (P = 0.012). Both linear and quadratic trands were noticed in the relative abundance of prophenoloxidase, lysozyme and penaeidin-3α genes mRNA transcript in the HP of shrimps fed Se-N supplemented diets. However, heat shock protein 70 gene transcript did not show any trend by supplementing Se-N in the experimental diets. In conclusion, based on the second-order polynomial regression analysis the optimum level of Se-N supplementation in a diet for P. vannamei juveniles was estimated to be 0.38 mg/kg diet. Furthermore, extra levels of dietary Se-N decreased growth rate and PO activity tha