A 8 weeks long feeding trial was carried out to investigate the effect of a potential probiotic bacterial strain, Lactobacillus plantarum, supplemented in diets on growth performance and immune responses of striped catfish (Pangasianodon hypophthalmus). The feeding trial had three treatments supplemented with 106, 107, and 108 CFU L. plantarum g-1 of feed, and one control group without probiotic supplementation. Fish fed with L. plantarum diets showed significantly higher growth performance compared to those supplemented with the control diets (p < 0.05) after feeding trial. The weight gain, daily weight gain, and specific growth rate of the treatment supplemented with 107 CFU g-1 was the highest. The survival rates were not significantly different between the treatments. In terms of immunity, fish blood was sampled for hematological parameters, lysozyme, and complement activity. Results also showed that hematological parameters (total white blood cells, monocytes, lymphocytes, neutrophils, and thrombocytes were significantly higher (p < 0.05) in the probiotic supplemented groups compared to those of the control group. Total lysozyme activity and complement activity was significantly higher in the probiotic supplemented groups than in the control group (p < 0.05), indicating higher immunity in the probiotic supplemented group. After completion of feeding trial, the fish were challenged with Edwardsiella ictaluri for evaluating the protection against injected bacteria. Accumulated mortalities after bacterial challenge were higher in the control group compared to the probiotic-fed group. The results suggest that L. plantarum has potential for improving the growth performance and modulating the immune response of the striped catfish larvae.