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Therapeutic resistance to DNA damage is a significant challenge in oncology. To gain insight into biological mechanisms that cause DNA damage resistance and to inform strategies for achieving synergy with therapeutic radiation, we performed parallel pooled genetic CRISPR-Cas9 screening for survival in high-risk head and neck squamous cell carcinoma (HNSCC) subtypes. Surprisingly, in addition to known mediators of radiotherapy response, including ATM, DNAPK, and NF-κB signaling, loss of JAK1 was identified as a driver of tumor cell radioresistance. Knockout of JAK1 in HNSCC increased cell survival by enhancing the DNA damage-dependent G2/M cell cycle arrest and delaying progression to radiation-induced mitotic catastrophe. In line with this finding, both JAK1 knockout and kinase inhibition with abrocitinib prevented subsequent formation of radiation-induced micronuclei. Loss of JAK1 function did not affect canonical CDK1 signaling but instead reduced activation of PLK1 and AURKA, two kinases with auxiliary roles in the regulation of G2 and M phase progression. Correspondingly, using both EdU labelling and live cell imaging techniques, JAK1 loss was found to cause prolonged metaphase, mitotic slippage, and progression to tetraploidy. Targeting the mitotic kinesin KIF18A with the small molecule sovilnesib exacerbated mitotic stress and enhanced the efficacy of radiation. These studies establish KIF18A inhibition as a strategy to counteract the protective G2/M cell cycle arrest induced by DNA damage and to thus enhance tumor cell sensitivity to radiation therapy.