Abstract: Tiger shrimp is one of Indonesias foreign export commodities. Tiger shrimp has decreased production due to various diseases. Ulva reticulata seaweed is known to have polysaccharide sulfate and secondary metabolites that can enhance immunity in shrimp. This study was conducted to analyze the potential of Ulva reticulata extract in increasing the immune response in tiger shrimp. The study design used a Completely Randomized Design (CRD) with 4 treatments with 3 replications. As a treatment, mixing Ulva reticulata extract on commercial feed with a dose of 0, 0.5, 1.0 and 1.5 g kg-1 feed for 14 days. The observation parameters consisted of hemocyte counts and phagocytic activity. The results showed the highest hemocytes and the highest phagocytic activity was shown in the treatment of 1.5 g kg-1 with total hemocytes of 3.0 x 105 cells/ml and the percentage of phagocytic activity was 77%.