Laminaria japonica is a type of brown algae widely consumed in Asian countries and contains many essential nutrients and exhibits anti-obesity, antioxidant, and anti-inflammatory effects. In this study, the antioxidant and immunomodulatory effects of a Laminaria japonica water extract (LJE) were investigated using an in vitro model. Mean total polyphenol content of LJE was 2.16±0.11 μg GAE/mg, and LJE dose-dependently inhibited ABTS radical activity but did not scavenge DPPH radicals. In addition, LJE enhanced nitric oxide (NO) production and upregulated the mRNA expressions of proinflammatory cytokines (i.e., tumor necrosis factor-α and interleukin-6) in RAW 264.7 cells. On the other hand, LJE inhibited NO production and downregulated proinflammatory cytokine mRNA levels in endotoxin-stimulated RAW 264.7 cells. Thus, our data show that LJE has moderate antioxidant activity and biphasic immunomodulatory effects on RAW 264.7 cells. In summary, the study indicates that LJE has potential therapeutic use as a novel biphasic immuno-modulator.