Osteoclasts are the only bone-resorbing cells in organisms and understanding their differentiation mechanism is crucial for the treatment of osteoporosis. In the present study, we investigated the effect of Thiamet G, an O -GlcNAcase specific inhibitor, on osteoclastogenic differentiation. Thiamet G treatment increased global O -GlcNAcylation in murine RAW264 cells and suppressed receptor activator of nuclear factor-κB ligand (RANKL)-dependent formation in tartrate-resistant acid phosphatase (TRAP)-positive multinuclear cells, thereby suppressing the upregulation of osteoclast specific genes. Meanwhile, knockdown of O -linked N -acetylglucosamine ( O -GlcNAc) transferase promoted the formation TRAP-positive multinuclear cells. Thiamet G treatment also suppressed RANKL and macrophage colony-stimulating factor (M-CSF) dependent osteoclast formation and bone-resorbing activity in mouse primary bone marrow cells and human peripheral blood mononuclear cells. These results indicate that the promotion of O -GlcNAc modification specifically suppresses osteoclast formation and its activity and suggest that chemicals affecting O -GlcNAc modification might potentially be useful in the prevention or treatment of osteoporosis in future.