In our previous studies, geraniin was reported to have a preventive effect in the rat model of tretinoin-induced osteoporosis. However, whether geraniin exhibits an inhibitory effect on bone resorption or on MMP-9 expression is not yet known. We present here our novel findings from in vitro experiments that geraniin (a) decreases the number of mature osteoclasts and pre-osteoclast in cultures, (b) reduces the osteoclastic fusion index, and (c) inhibits the resorption areas and resorption pits. We also report that geraniin suppresses the mRNA and protein expression levels of MMP-9. These results demonstrate that geraniin has an inhibitory effect on the bone-absorption ability of osteoclasts in vitro, and the mechanisms may be closely associated with the downregulation of mRNA and protein expression of MMP-9.