Fucoidan is a polysaccharide purified from the brown seaweed Fucus vesiculosus. Although some effects of fucoidan on immune functions have been elucidated, there have been no studies concerning the immunomodulatory effects of fucoidan on dendritic cells (DCs), which are powerful antigen-presenting cells. In this study, fucoidan increased the viability of DCs, the production of interleukin-12 and tumor necrosis factor-alpha, and the expression of major histocompatibility complex class I, class II, CD54, and CD86 molecules. Furthermore, fucoidan-treated DCs showed decreased antigen uptake and increased proliferation of allogeneic splenocytes. In a study of the transcriptional regulation effects of fucoidan, translocation of p65 molecules of nuclear factor-kappaB (NF-kappaB) from the cytosol to the nucleus was clearly observed in fucoidan-treated DCs. Taken together, the results suggest that fucoidan has immunostimulating and maturing effects on DCs, via a pathway involving at least NF-kappaB.