SMILE (small heterodimer partner interacting leucine zipper protein) has been identified as a corepressor of the glucocorticoid receptor, constitutive androstane receptor, and hepatocyte nuclear factor 4alpha. Here we show that SMILE also represses estrogen receptor-related receptor gamma (ERRgamma) transactivation. Knockdown of SMILE gene expression increases ERRgamma activity. SMILE directly interacts with ERRgamma in vitro and in vivo. Domain mapping analysis showed that SMILE binds to the AF2 domain of ERRgamma. SMILE represses ERRgamma transactivation partially through competition with coactivators PGC-1alpha, PGC-1beta, and GRIP1. Interestingly, the repression of SMILE on ERRgamma is released by SIRT1 inhibitors, a catalytically inactive SIRT1 mutant, and SIRT1 small interfering RNA but not by histone protein deacetylase inhibitor. In vivo glutathione S-transferase pulldown and coimmunoprecipitation assays validated that SMILE physically interacts with SIRT1. Furthermore, the ERRgamma inverse agonist GSK5182 enhances the interaction of SMILE with ERRgamma and SMILE-mediated repression. Knockdown of SMILE or SIRT1 blocks the repressive effect of GSK5182. Moreover, chromatin immunoprecipitation assays revealed that GSK5182 augments the association of SMILE and SIRT1 on the promoter of the ERRgamma target PDK4. GSK5182 and adenoviral overexpression of SMILE cooperate to repress ERRgamma-induced PDK4 gene expression, and this repression is released by overexpression of a catalytically defective SIRT1 mutant. Finally, we demonstrated that ERRgamma regulates SMILE gene expression, which in turn inhibits ERRgamma. Overall, these findings implicate SMILE as a novel corepressor of ERRgamma and recruitment of SIRT1 as a novel repressive mechanism for SMILE and ERRgamma inverse agonist.