Human adipose-derived stem cells (hADSCs) are an attractive source for cell therapies, because they can be obtained from aspirated adipose tissues with the capacity of proliferation and differentiation into several mesenchymal lineages under certain conditions. Sulfated polysaccharides, including heparin, modulate osteogenic differentiation of stem cells through the regulation of growth factor binding and signaling pathways. Here, we examined the effects of the sulfated polysaccharide fucoidan on osteogenic differentiation of hADSCs. Strikingly, fucoidan treatment resulted in increased alkaline phosphatase (ALP) activity and alizarin red and von Kossa staining. At the molecular level, fucoidan treatment enhanced the expression of osteogenesis-specific marker genes, including ALP, osteopontin, type I collagen, Runt-related transcription factor 2, and osteocalcin. Furthermore, fucoidan also promoted the osteogenic differentiation of another mesenchymal cell lineage, human amniotic fluid stem cells. These findings strongly suggest that fucoidan enhances osteogenic differentiation of hADSCs and possibly other mesenchymal cell lineages, indicating that it may be a potential candidate for promoting bone regeneration.