This work highlighted the impact of dry concentrations (0, 1, 2, and 4 g/kg diet) of the marine diatom species Skeletonema costatum(SK 0%, SK0.1%, SK0.2%, and SK0.4%, respectively) as an aquafeed additive on the postlarvae of Pacific whiteleg shrimp (Litopenaeusvannamei). During an 8-week feeding trial, the impact of S. costatum on growth, digestive enzymes, antioxidant activities,immunity-associated gene expressions, and immunological responses of Pacific white shrimp was investigated. The protein, lipid,and carbohydrates of S. costatum were 33.50%, 35.95%, and 11.87%, respectively. Shrimp reared in group SK 0.4% exhibited asignificant improvement in growth performance indicators, compared to the other groups. The results showed that, compared tothe control group (SK 0%), shrimp-fed S. costatum supplemented groups (SK0.1%, SK0.2%, and SK0.4%) showed significantly(p <0:05) higher amylase activities. Compared to the control group (SK0%), shrimp-fed S. costatum groups (SK 0.1%, SK0.2%, andSK 0.4%) demonstrated major significant (p<0:05) improvement in catalase and superoxide dismutase values. Shrimp reared ingroup SK 0.4% showed a significant (p<0:05) improvement in lipase and lysozyme activities. The increasing levels of S. costatumdietary supplementation significantly (p<0:05) increase the relative gene expression of the p53-like protein isoform delta (p53)gene. SK0.4% revealed a major significant (p<0:05) increase in the relative gene expression of the prophenoloxidase (PPO1) andperoxiredoxin (Prx) genes. This study highlights the potential of S. costatum as a promising aquafeed additive enhancing growthperformance and immunity of Pacific whiteleg shrimp health, opening the way for more sustainable shrimp aquaculture practices.