The DNA damage repair kinase ATM is phosphorylated by the NF-κB pathway kinase IKKα, resulting in enhanced DNA damage repair through the nonhomologous end-joining pathway. Thus, inhibition of IKKα enhances the efficacy of cancer therapy based on inducing DNA damage. Here, we found a role for the IKK regulatory subunit NEMO in DNA damage repair mediated by ATM and IKKα. Exposure to damaging agents induced the interaction of NEMO with a preformed ATM-IKKα complex, which was required to target active ATM and IKKα to chromatin for efficient DNA damage repair but not for activating ATM. Recognition of damaged DNA by the IKKα-NEMO-ATM complex was facilitated by the interaction between NEMO and histones and depended on the ADP ribosylation of histones by the enzyme PARP1. NEMO-deficient cells showed increased activity of the kinase ATR, and inhibition of ATR potentiated the effect of chemotherapy in cells lacking NEMO or IKKα. Bioinformatic analysis of colorectal cancer datasets demonstrated that the expression of genes encoding IKKα, NEMO, and ATM correlated with poor patient prognosis, suggesting that the mechanism linking these three elements may be clinically relevant.