PPAR 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 PPAR transactivation remains unresolved. In this report, we cloned and characterized a novel PPAR 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-PPAR interaction by GST-pull-down and co-immunoprecipitation assays indicated that CCPG interacts with the hinge region of PPAR in a ligand-independent manner. However, mutation of four motifs of LXXLL to LXXAA in CCPG does not compromise its interaction with PPAR, suggesting LXXLL motif is not required for the interaction. GST pull-down assays showed that CCPG binds to RXR and ER independent of their ligands, but not to TR. CCPG coactivates PPAR in PPRE reporter assays and the N-terminus (amino acids 1-561) of CCPG acts to significantly augment the transactivation of PPAR, while the C-terminus (amino acids 562-786) represses PPAR activity indicating the N-terminus possesses the activation domain. Using adenoviral mediated system, we also revealed that overexpression of CCPG promoted differentiation of OP9 preadipocyte into adipocyte, and knockdown of CCPG by RNAi 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 PPAR dependent manner.