Glucocorticoids, such as dexamethasone, have been used as in vitro inducers of adipogenesis. However, the roles of the glucocorticoid receptor (GR) in adipogenesis have not been well characterized yet. Here, we show that inhibition of GR activity using the GR antagonist RU486 prevents human mesenchymal stem cell and mouse embryonic fibroblast (MEF) differentiation into adipocytes. Moreover, in MEFs isolated from GR knockout (GR(null)) and GR(dim) mice deficient in GR DNA-binding activity, adipogenesis was blocked. We identified glucocorticoid response element sites in the first intron of KLF15 by bioinformatical promoter analysis and confirmed their functional relevance by demonstrating GR interaction by chromatin immunoprecipitation. Moreover, transfection of MEFs with siRNA for KLF15 significantly attenuated the expressions of adipogenic-marker genes and the lipid accumulation. Our results provide a new mechanism for understanding glucocorticoids-dependent adipogenesis and that GR promotes adipogenesis via KLF15 gene expression as a transcriptional direct target.