--- - |2- Our findings identify orphan nuclear receptor ERRγ as a novel negative regulator of osteoclastogenesis and further reveals its protective role on inflammatory bone loss in vivo. Estrogen‐related receptor γ (ERRγ) is an orphan nuclear receptor that plays an important role in various metabolic processes under physiological and pathophysiological conditions. Here, we report that ERRγ functions as a negative regulator in receptor activator of nuclear factor κΒ ligand (RANKL)–induced osteoclast differentiation. We observed that ERRγ was strongly expressed in osteoclast precursors, bone marrow–derived macrophages (BMMs) while its expression was significantly reduced by RANKL during osteoclastogenesis. Overexpression of ERRγ in BMMs suppressed the formation of multinucleated osteoclasts and attenuated the induction of c‐Fos and nuclear factor of activated T cells c1, which are critical modulators in osteoclastogenesis. Similarly, the treatment of ERRγ agonists, N‐(4‐(diethylaminobenzylidenyl)‐N'‐(4‐hydroxybenzoyl)‐hydrazine (DY131) or GSK4716, also inhibited osteoclast generation and the expression of these key modulators. On the other hand, shRNA‐mediated knockdown of ERRγ accelerated the formation of bone‐resorbing cells and the expression of osteoclastogenic markers. Forced expression of ERRγ blocked RANKL‐stimulated phosphorylation of the nuclear factor κB (NF‐κB) inhibitor IκBα and suppressed NF‐κB transcriptional activity induced by RANKL or the NF‐κB subunit p65. Furthermore, by employing a pharmacological approach, we showed that the ERRγ agonist DY131 protected against inflammatory bone loss induced by lipopolysaccharide in vivo. Together, our findings reveal that ERRγ is a pivotal regulator in RANKL‐mediated osteoclastogenesis and suggest that ERRγ may have potential as a therapeutic target for pathological bone loss. - 'Journal of Cellular Physiology, Volume 234, Issue 2, Page 1659-1670, February 2019. '