The ataxia telangiectasia mutated (ATM) protein kinase is a critical component of a DNA-damage response network configured to maintain genomic integrity. The abundance of an essential downstream effecter of this pathway, the tumor suppressor protein p53, is tightly regulated by controlled degradation through COP1 and other E3 ubiquitin ligases, such as MDM2 and Pirh2; however, the signal transduction pathway that regulates the COP1-p53 axis following DNA damage remains enigmatic. We observed that in response to DNA damage, ATM phosphorylated COP1 on Ser³⁸⁷ and stimulated a rapid autodegradation mechanism. Ionizing radiation triggered an ATM-dependent movement of COP1 from the nucleus to the cytoplasm, and ATM-dependent phosphorylation of COP1 on Ser³⁸⁷ was both necessary and sufficient to disrupt the COP1-p53 complex and subsequently to abrogate the ubiquitination and degradation of p53. Furthermore, phosphorylation of COP1 on Ser³⁸⁷ was required to permit p53 to become stabilized and to exert its tumor suppressor properties in response to DNA damage.