A 49-day rearing trial was conducted to investigate the effects of Pseudoalteromonas espejiana W115 on digestive and immune–related enzyme activities in larval and juvenile Yesso scallop Patinopecten yessoensis. Larvae (day 2 post-fertilization) (day 2 PF) were distributed into three groups and provided with a diet only of microalgae (control group) and supplemented with W115 at final concentrations of 1 × 10⁴ and 1 × 10⁶ cells/mL, respectively. Results showed that umbo larvae (day 13 PF) and/or crawling-pediveliger larvae (day 28 PF) and/or juveniles (day 51 PF) fed with W115 supplementation diet at 10⁴ cells/mL showed a significant increase in pepsin, amylase and cellulase activities compared to those fed the diet of microalgae, however, no significant difference was observed between the 10⁶ cells/mL supplementation and control groups at days 13, 28 and 51 PF. The lysozyme and superoxide dismutase activities of umbo larvae and/or crawling-pediveliger larvae and juveniles treated with 10⁴ and/or 10⁶ cells/mL W115 supplementations were significantly higher than those in the control group, while there were no significant differences in enzyme activities in larvae and juveniles treated with two doses of W115 at days 13, 28 and 51 PF. No significant difference in catalase activity was observed among the three groups at days 13, 28 and 51 PF. In addition, there was increased survival in W115-amended treatments after juveniles were challenged with Vibrio splendidus. The results suggest that a 10⁴ cells/mL supplementation of W115 can be useful in Yesso scallop larviculture.