The transcription factor nuclear factor κB (NF-κB) regulates various cellular processes such as inflammation and apoptosis. The NF-κB essential modulator (NEMO/IKKγ) is indispensable for NF-κB activation by diverse stimuli including genotoxic stress. Here, we show that NEMO linear ubiquitination on K285/309 is critical for genotoxic NF-κB activation. The E3 ligase linear ubiquitin chain assembly complex (LUBAC) facilitates NEMO linear ubiquitination upon genotoxic stress. Inhibiting LUBAC function interrupts the genotoxic NF-κB signalling cascade by attenuating the activation of IKK and TAK1 in response to DNA damage. We further show that the linear ubiquitination of NEMO is a cytoplasmic event, potentially downstream of NEMO nuclear exportation. Moreover, ELKS ubiquitination appears to facilitate linear ubiquitination of NEMO through stabilizing NEMO:LUBAC association upon DNA damage. Deubiquitinating enzyme CYLD inhibits NEMO linear ubiquitination, possibly by disassembling both K63-linked and linear polyubiquitin. We also found that abrogating linear ubiquitination of NEMO significantly increased genotoxin-induced apoptosis, resulting in enhanced sensitivity to chemodrug in cancer cells. Therefore, LUBAC-dependent NEMO linear ubiquitination is critical for genotoxic NF-κB activation and protects cells from DNA damage-induced apoptosis.