Post-translational modification of proteins by the ubiquitin-like molecule SUMO-1 regulates their stability and activity with crucial implications for many cellular processes. Here we show that p63alpha, but not p63beta and gamma, is sumoylated in vitro and in vivo at a single lysine residue, K637, in the post-SAM domain. SUMO-1 attachment targets DeltaNp63alpha for proteasome mediated degradation while it does not influence p63alpha intracellular localization, as wild-type protein and a mutant carring the K637 mutated into arginine (K637R), have the same nuclear localization. Four natural p63 mutations, falling within the SAM and post-SAM domain of p63alpha, were found to be altered in their sumoylation capacity. The transcriptional activities of the natural mutants and of K637R were strongly increased compared to that of wild type p63, suggesting that sumoylation has a negative effect on p63 driven transcription. The findings that DeltaNp63alpha protein levels are regulated by SUMO-1 and that this regulation is altered in natural p63 mutants, suggest that SUMO conjugation to p63 plays a critical role in regulating the biological activity of p63.