Salinity is the most variable ecological parameter in the lagoon with daily and seasonal variations. The changes in growth, serum biochemistry and antioxidant enzymes activities of Cardisoma armatum were examined during 84-day exposure to five different sublethal salinities (0, 5, 10, 15 and 20 ppt). At the start of the trial, the crab initial average body weight was not significantly different (P > 0.05). The highest weight gain was recorded in 15 ppt (47.40±1.01 g), followed by 20 ppt, 10 ppt and 5 ppt with no significant differences between them. At the end of 84 days experiment, the crab exhibited the lowest body weight growth (48.29 %) at 0 ppt and the highest (81.04±1.08%) at 10 ppt. The 10 ppt treatment had the highest specific growth rates (0.31±0.11 %/day), followed by 5 ppt, 20 ppt, 15 ppt, and then 0 ppt treatments. Aspartate and alanine aminotransferases significantly decreased after 0 ppt, but mean serum protein value increased with salinity increase. Greater activities of catalase, superoxide dismutase and malondialdehyde were recorded in 5-20 ppt. All the antioxidant enzyme activities (with the exception of glutathione) showed significant differences. Thus, deviation from the brackish water condition adversely affects the growth and immune functions of the gecarcinid, resulting in population decline in reclaimed wetlands.