A new naphthalene glycoside was isolated from the leaves and stems of Chimaphila umbellata Barton. Its chemical structure was elucidated to be 2,7-dimethyl-1,4-dihydroxynaphthalene-1-O-β-D-glucopyranoside (DMDHNG), based on spectroscopic evidence. DMDHNG significantly inhibited the receptor activator of nuclear factor-κB ligand (RANKL)-induced tartrate-resistant acid phosphatase (TRAP) activity and the formation of multinucleated osteoclasts in a dose-dependent manner. In addition, the new glycoside inhibited the RANKL-induced mRNA expression of osteoclast-associated genes that encode TRAP, cathepsin K, and another transcription factor-nuclear factor of activated T-cells c1. We believe that the inhibitory effects of DMDHNG on the osteoclast differentiation may be exploited for a therapeutic benefit.