Sargassum microcystum is a common brown seaweed (Phaeophyta) distributing around the coast of Kien Giang Province, Vietnam. Crude polysaccharide from S. microcystum was extracted using hot-water within 3 h then analyzed for protein, total phosphorus, glucose, L-fucose, sulfate, and phlorotamin. Antioxidant activity is also evaluated through free radicals scavenging activity. Then, the hot-water extract was incorporated with commercially available artificial pellet feed to examine its effectiveness on growth performance and survival of striped catfish Pangasianodon hypophthalmus in outdoor experimental condition. For the growth performance trial, one hundred experimental fish (1~2 g) were held in 0.5 ton, then fed relative diets to satiation for 60 days. Four dietary treatments were tested, each with three replicates. Fish were fed at different levels of polysaccharide extract at 0.2, 0.4, and 0.6%. Fish were fed the diet without polysaccharide extract served as control group. Growth performance indices as growth rate, percentage of weight gain, feed efficiency, survival rate, and total harvested weight were evaluated at the end of experiment. Some crucial water quality parameters as temperature, pH, dissolved oxygen, N-ammonia, and N-nitrite were also tested weekly. The results indicated that striped catfish P. hypophthalmus that being fed the diets incorporating with hot-water extracts at 0.2~0.4% had significantly higher growth performance than that of control after 60 days of feeding (p< 0.05). However, no significant difference in survival rate and food conversion rate among treatments was found (p> 0.05). Therefore, it is concluded that polysaccharide extracted from brown seaweed S. microcystum could be considered to be a growth-promoting factor in striped catfish P. hypophthalmus culture. To the best of our knowledge, this is the first study in application of brown seaweed extract collecting in Mekong Delta, Vietnam on growth and survival of striped catfish P. hypophthalmus.