Constitutive photomorphogenic 1 (COP1) is a p53-targeting E3 ubiquitin ligase that is downregulated by DNA damage through mechanisms that remain obscure. Here, we report that COP1 is not downregulated following DNA damage in 14-3-3σ null cells, implicating 14-3-3σ as a critical regulator in the response of COP1 to DNA damage. We also identified that 14-3-3σ, a p53 target gene product, interacted with COP1 and controlled COP1 protein stability after DNA damage. Mechanistic studies revealed that 14-3-3σ enhanced COP1 self-ubiquitination, thereby preventing COP1-mediated p53 ubiquitination, degradation, and transcriptional repression. In addition, we found that COP1 expression promoted cell proliferation, cell transformation, and tumor progression, manifesting its role in cancer promotion, whereas 14-3-3σ negatively regulated COP1 function and prevented tumor growth in a mouse xenograft model of human cancer. Immunohistochemical analysis of clinical breast and pancreatic cancer specimens demonstrated that COP1 protein levels were inversely correlated with 14-3-3σ protein levels. Together, our findings define a mechanism for posttranslational regulation of COP1 after DNA damage that can explain the correlation between COP1 overexpression and 14-3-3σ downregulation during tumorigenesis.