The p53 homolog p63 is a transcriptional activator. Here, we describe the identification of an HMG1-like protein SSRP1 as a co-activator of p63. Over expression of wild-type, but not deletion mutant, SSRP1 remarkably enhanced p63gamma-dependent luciferase activity, G1 arrest, apoptosis and expression of endogenous PIG3, p21(Waf1/cip1) and MDM2 in human p53-deficient lung carcinoma H1299 cells and mouse embryonic fibroblasts. Also, SSRP1 interacted to p63gamma in vitro and in cells, and resided with p63gamma at the p53-responsive DNA element sites of the cellular endogenous MDM2 and p21(Waf1/cip1) promoters. Moreover, N-terminus-deleted p63 (DeltaN-p63) bound to neither SSRP1 nor its central domain in vitro. Accordingly, SSRP1 was unable to stimulate DeltaN-p63-mediated residual luciferase activity and apoptosis in cells. Finally, the ectopic expression of the central p63-binding domain of SSRP1 inhibited p63-dependent transcription in cells. Thus, these results suggest that SSRP1 stimulates p63 activity by associating with this activator at the promoter.