The transcriptional co-activator p300 has been reported to regulate the tumor suppressor p53 and its ortholog p73. Here we describe a study showing that this coactivator also regulates the transcriptional function of p63. p300 bound to the N-terminal domain of p63gamma, and p63gamma bound to the N terminus of p300 in vitro and in cells. p300, but not its acetylase-defective mutant AT2, stimulated p63gamma-dependent transcription and induction of p21 in cells, consequently leading to G1 arrest. Inversely, the deltaN-p63gamma isoform as well as p300AT2 inhibited the induction of p21 by p63gamma. These results suggest that p300 regulates p63-dependent transcription of p21.