p73, a homolog of the tumor suppressor p53, transactivates many p53 target genes, leading to apoptosis or cell-cycle arrest. p73 has recently been reported to play an important role in tumor suppression in a mouse model. Here, we show that Pirh2 physically interacted with p73 and downregulated p73 function through its E3 ligase activity. Pirh2 promoted p73 ubiquitination in vivo and in vitro. Intriguingly, Pirh2 primarily used K63-linked chains to ubiquitinate p73 in vitro, but in vivo, Pirh2 utilized K11-, K29-, K48-, and K63-linked chains to promote p73 ubiquitination. Depletion of Pirh2 by siRNA significantly reduced the ubiquitination of p73 in p53 null cells. Ectopic expression of Pirh2 repressed p73-dependent transcriptional activity, but the levels of p73 were not decreased. We consistently showed that ablation of endogenous Pirh2 restored p73-mediated transactivational activity. We found that Pirh2 repressed p73 transcriptional activity by directly inhibiting the p73 transcript, and p73 repression by Pirh2 was required for p73-dependent transcriptional activity and G(1) arrest but not for apoptosis. This study provides evidence that the ubiquitination of p73 mediated by Pirh2 represents an important pathway for controlling the suppressive function of p73. Furthermore, the data suggest a link between the transcriptional activity of p73 and its ubiquitination.