Ocean warming affects the important coastal communities including seagrass beds. The species interaction of seagrass beds is unique in that grazers can positively affect seagrass by feeding on epiphytic algae which compete with seagrass. However, the effects of seawater warming on these interactions are still unknown. This study examines the effects of ocean warming in two seasons by conducting laboratory experiments. We hypothesized that the increasing temperature strengthens the grazing effect on epiphytic algae, and as a result, it positively affects the growth of seagrass. The surfgrass Phyllospadix iwatensis and the snail Lacuna uchidai were used for this study. The total amount of epiphytes and their size structure, and the shoot growth of P. iwatensis were measured as response variables. Temperature increase affected the grazing effect on epiphytic algae although the direction of the effects differed among seasons. The indirect effects of snails on surfgrass were not detected possibly due to the absence of competition between epiphytic algae and P. iwatensis. My study highlighted the needs of observing both direct and indirect effects of temperature warming on coastal communities, and of examining their seasonal variation.