Abstract We identified a previously unknown integrin, α9β1, on OCLs and their precursors. Antibody to α9 inhibited OCL formation in human marrow cultures, and OCLs from α9 knockout mice had a defect in actin ring reorganization and an impaired bone resorption capacity. Introduction: Integrins play important roles in osteoclast (OCL) formation and function. Mature OCLs mainly express αvβ3 integrin, a heterodimer adhesion receptor that has been implicated in osteoclastic bone resorption. We identified ADAM8, a disintegrin and metalloproteinase, as a novel stimulator of OCL differentiation and showed that the disintegrin domain of ADAM8 mediated its effects on OCL formation. Because the disintegrin domain of ADAM8 does not bind Arg-Gly-Asp (RGD) sequences, we determined which integrin bound ADAM8 and characterized its role in OCL formation and activity. Materials and Methods: Chinese hamster ovary cells (CHO) expressing different integrin subunits were tested for their capacity to bind the disintegrin domain of ADAM8. Mouse or human bone marrow cells and purified OCL precursors were tested for α9β1 integrin expression by Western blot, immunocytochemistry, and real-time RT-PCR. A monoclonal antibody to human α9 was used to block α9β1 on OCL precursors stimulated by 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] or RANKL. Vertebrae of 7-day-old α9−/− mice and wildtype (WT) littermates were compared using bone histomorphometry and 3D μCT analysis. Results: α9 integrin was expressed by mouse and human bone marrow–derived OCLs and their precursors. Importantly, the anti-α9 antibody inhibited human OCL formation stimulated by 1α,25(OH)2D3 or RANKL dose-dependently. Furthermore, analysis of OCLs formed in marrow cultures from α9−/− mice showed that the OCLs formed were more contracted and formed significantly less bone resorption pits on dentin slices. Histologic analysis of α9−/− vertebrae showed thickened trabecular regions and retained cartilage within vertebral bodies of α9−/− mice. 3D μCT analysis of α9−/− vertebrae also showed a significant increase in trabecular bone volume/total tissue volume and a tendency for decreased trabecular separation compared with WT mice. Conclusions: These results support a previously unknown role for α9β1 integrin in OCL formation and function.