The aim of this study was to examine whether (1) severe changes in salinity produced increased stress, and (2) vitamin C supplementation might reduce the observed damage in Nile tilapia. The parameters measured included condition factor, survival rate, and gene expression of catalase (CAT), heat shock protein 70 (HSP70), glutathione reductase (GSR), glutathione synthase (GSS), and glutathione peroxidase (GPx). The investigation was conducted with 160 Nile tilapia divided into four treatment groups: freshwater; 7 or 21 parts per thousand (‰) salinity, all fed a basal diet; as well as a fourth treatment group consisting of fish kept at 21‰ salinity fed a diet supplemented with vitamin C (1500 mg/kg). For gene expression analysis, liver samples were collected after 24 h or after 14 d. After 24 h, fish raised in 21‰ salinity and fed with the diet supplemented with vitamin C showed similar GPx expression as the control freshwater group. GSS expression in 21‰ salinity was similar to fish exposed to 7‰ salinity. Nile tilapia exposed to 21‰ salinity without vitamin C supplementation exhibited the highest HSP70 gene expression levels after 24 h. After 14-dtreatment, the lowest survival rate was observed in the 21‰ salinity group. After 14 d, the highest expression of GPx and GSR levels was detected in fish in the 21‰ salinity group that received vitamin C. Data indicate that vitamin C supplementation enhanced the expression of genes related to antioxidant capacity in Nile tilapia exposed to higher salinity, thereby increasing protection against the oxidative effects induced by high water salinity.</AB>.