β-Glucan is a polysaccharide that is widely used as an adductive in fish feed to facilitate immune stimulation. This study aimed to investigate the effect of β-glucan on immune responses in the Manila clam Ruditapes philippinarum. For this purpose, three groups of R. philippinarum were exposed to 0%, 0.1%, or 1% β-glucan in sea water for 1 hr/day for 2 weeks using an immersion method. Thereafter, two immune parameters-phagocytic rate and antibacterial activity-were measured. R. philippinarum exposed to 1% β-glucan showed an approximate 30% significant increase in phagocytic rate. In addition, β-glucan significantly limited the growth of the pathogenic bacteria Vibrio tapetis, V. parahaemolyticus, and V. ordalii. Moreover, the mortality rates of β-glucan-treated clams decreased during a 17-day experiment. Our study suggests that treatment with β-glucan significantly increases the immune responses in R. philippinarum, and that immersion is a simple and effective method for immune stimulation in this species.