CD44, an adhesion molecule that binds to the extracellular matrix, primarily to hyaluronan (HA), has been implicated in cancer cell migration, invasion, and metastasis. CD44 has also recently been recognized as a marker for stem cells of several types of cancer. However, the roles of CD44 in the development of bone metastasis are unclear. Here, we addressed this issue by using bone metastatic cancer cell lines, in which CD44 was stably knocked down. Tumor sphere formation and cell migration and invasion were significantly inhibited by CD44 knockdown. Furthermore, the downregulation of CD44 markedly suppressed tumorigenicity and bone metastases in nude mice. Of note, the number of osteoclasts decreased in the bone metastases. Microarray analysis revealed that the expression of HA synthase 2 was downregulated in CD44-knockdown cells. The localization of HA in the bone metastatic tumors was also markedly reduced. We then examined the roles of CD44-HA interaction in bone metastasis using 4-methylumbelliferone (4-MU), an inhibitor of HA synthesis. 4-MU decreased tumor sphere and osteoclast-like cell formation in vitro. Moreover, 4-MU inhibited bone metastases in vivo with reduced number of osteoclasts. These results collectively suggest that CD44 expression in cancer cells promotes bone metastases by enhancing tumorigenicity, cell migration and invasion, and HA production. Our results also suggest the possible involvement of CD44-expressing cancer stem cells in the development of bone metastases through interaction with HA. CD44-HA interaction could be a potential target for therapeutic intervention for bone metastases.