We previously demonstrated the suppressive effects of regulatory T cells (Treg cells) on osteoclast differentiation in vitro. In this article, we show that blood markers of bone resorption inversely correlate with the amount of circulating Treg cells in healthy controls and rheumatoid arthritis patients, further suggesting that Treg cells may control bone destruction in vivo. Indeed, bone marrow from Foxp3-transgenic (Foxp3tg) mice fully protected human TNF transgenic (hTNFtg) mice from TNF-α–induced bone destruction, whereas Foxp3-deficient bone marrow enhanced local and systemic bone loss. The same protective effect was also obtained by treating hTNFtg mice with the CD28 superagonist mAb (CD28 SA), which increased Treg cell numbers. In both models, bone protection by Treg cells was associated with reduced osteoclast numbers, resulting in less bone-resorbing activity. Reduced osteoclast numbers were not caused by an intrinsic defect in osteoclast differentiation because osteoclast precursors from hTNFtg/Foxp3tg chimeras responded normally to M-CSF and receptor activator of NF-κB ligand. Although a decrease in the clinical signs of arthritis was observed in Foxp3tg bone marrow-transferred and CD28 SA-treated hTNFtg mice, the bone-protective effect of Treg cells was independent of the suppression of inflammation, as demonstrated by the increased systemic bone density observed in wild-type mice treated with CD28 SA. This work demonstrated that increasing Treg cell numbers improved clinical signs of arthritis and suppressed local and systemic bone destruction. Thus, enhancing the activity of Treg cells would be beneficial for the treatment of inflammation-induced bone loss observed in rheumatoid arthritis.