Milkfish is the top-most-produced and biggest finfish aquaculture industry in the Philippines. However, its nursery aquaculture production is affected by bacterial diseases. Vibriosis disease, caused by Vibrio spp. has been linked to low production in milkfish nursery operations. Ulvan, a sulfated polysaccharide known for its bioactive properties such as growth-promoting and immunomodulating, was extracted from Ulva intestinalis and used to supplement milkfish early juvenile diets to improve growth performance, increase expression of immune-related genes, and improve survival against Vibrio harveyi. Four experimental diets, including a control with no ulvan inclusion and three experimental diets (10, 50, and 100 mg kg-1 ulvan inclusion), were formulated and evaluated in milkfish early juveniles (0.0146 g mean weight). The test was run for 30 days following a completely randomized design with four treatments in triplicate. Weight gain (%), feed conversion efficiency (%), specific growth rate (%), and survival (%) were improved by ulvan supplementation, with 10 and 50 mg kg-1 inclusion being statistically significant (One-Way ANOVA, p<0.05) against the control (Table 1). 10 mg kg-1 ulvan inclusion also increased the differential gene expression of immune-related genes (IL6, TLR5, and LEAP2) by 84.5, 91.9, and 75.4 times, respectively, compared with the control (Independent samples T-test, p<0.01) (Table 2). The differential gene expression analysis results were also supported by the V. harveyi bacterial challenge test, showing that 10 mg kg-1 ulvan inclusion improved milkfish survival by 56.7% compared with the control (Independent samples T-test, p<0.01) 12 days post-exposure (Table 2). The data from this study suggests that ulvan inclusion at 10 mg kg-1 can significantly promote growth, immune activation, and survival of milkfish against vibriosis caused by V. harveyi. To our knowledge, this study is also the first to elucidate the effects of ulvan on milkfish growth performance and immunity.