Peroxisome proliferator-activated receptor gamma (PPARgamma) plays essential roles in adipogenesis by transcriptionally regulating adipocyte-specific genes through recruitment of coregulators including coactivators and corepressors. However, the precise repertoire of coactivators required for PPARgamma transactivation remains unresolved. In this report, we cloned and characterized a novel PPARgamma interacting protein, constitutive coactivator of PPARgamma (CCPG), which is expressed in multiple adult tissues and throughout embryonic development. CCPG is localized in nucleus and contains four LXXLL motifs, which are characteristic for nuclear receptor coactivators. A delineation of CCPG-PPARgamma interaction by glutathione-S-transferase pull-down and coimmunoprecipitation assays indicated that CCPG interacts with the hinge region of PPARgamma in a ligand-independent manner. However, mutation of four motifs of LXXLL to LXXAA in CCPG does not compromise its interaction with PPARgamma, suggesting LXXLL motif is not required for the interaction. Glutathione-S-transferase pull-down assays showed that CCPG binds to retinoic X receptor-alpha and estrogen receptor-alpha independent of their ligands, but not to thyroid hormone receptor-beta. CCPG coactivates PPARgamma in PPAR response element reporter assays, and the N terminus (amino acids 1-561) of CCPG acts to significantly augment the transactivation of PPARgamma, whereas the C terminus (amino acids 562-786) represses PPARgamma activity, indicating the N terminus possesses the activation domain. Using an adenoviral-mediated system, we also revealed that overexpression of CCPG promoted differentiation of OP9 preadipocyte into adipocyte, and knockdown of CCPG by RNA interference blocked this process, as examined by Oil Red O staining and Western blots of adipocyte-specific protein, adiponectin, and perilipin. Taken together, our data indicate that CCPG is a bona fide coactivator and promotes adipogenesis in a PPARgamma-dependent manner.