We report for the first time the expression of estrogen receptor-related receptor alpha, ERR, in fetal and adult rat chondrocytes in growth plate and articular cartilage and in the rat chondrogenic cell line C5.18 cells in vitro. ERR mRNA and protein were expressed from proliferating chondrocyte to mature chondrocyte stages. We show that over-expressing ERR in C5.18 cell cultures induces an increase in Sox9 expression, a master gene in cartilage formation. In parallel, we report Sox9 promoter regulation by ERR, in C5.18 cells. To assess a functional role for ERR in chondrogenesis, its expression was blocked by antisense oligonucleotides in C5.18 cell cultures and this led to inhibition of cartilage formation associated with down-regulation of Sox9 and Ihh expression and maturation of proliferating chondrocytes into hypertrophic chondrocytes in vitro. Together these results implicate ERR in the formation and maintenance of cartilage and also suggest that agonists and antagonists of ERR may be useful as therapeutic agents in a wide variety of diseases affecting cartilage and joints.