Asian sea bass (Lates calcarifer) an important marine species of high economic value and excellent meat quality, has suffered great losses due to disease in high-density aquaculture. The macroalgae, Gracilaria pygmaea, which include various bioactive compounds, may serve as an immunostimulant in the aquaculture industry. This study aimed to assess the effect of dietary fish meal replacement with Gracilaria pygmaea meal on immunity, activity of liver antioxidant enzymes, intestinal tissue, lysozyme gene activity, and IGF-I gene activity in Asian sea bass (Lates calcarifer). To this end, 120 individuals of Asian sea bass with an average weight of 28±0.5 g were divided into four treatments and three replicates and kept in 12 tanks (10 fish per 300-liter tank). Blends of soybean meal and fish meal were used as a Control diet (C). Experimental diets were prepared to substitute the fish meal with 3 (GL3), 6 (GL6), and 9% (GL9) of G. pygmaea in the basal diet. Fish were fed three times daily for six weeks. Dietary supplementation of G. pygmaea led to significantly increase in total immunoglobulin level compared to the control. The inclusion of G. pygmaea in the diet did not affect the antioxidant status of the fish. The histological analysis showed that fish of all groups exhibited normal morphology of anterior intestine and pyloric caeca. The obtained results showed that fish of GL9 and the control groups had the highest IGF-1 mRNA transcript abundance in the liver compared to the other groups. The changes of lysozyme expression noticed among the groups were statistically insignificant. Overall, the results obtained in this study indicated that dietary G. pygmaea did not cause adverse effects on immune status, antioxidant status, intestinal morphology, and lysozyme gene activity in Asian sea bass.