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.