Abstract Although many patients with colorectal cancer initially respond to the chemotherapeutic agent oxaliplatin, acquired resistance to this treatment remains a major challenge to the long-term management of this disease. To identify molecular targets of oxaliplatin resistance in colorectal cancer, we performed an shRNA-based loss-of-function genetic screen using a kinome library. We found that silencing of ataxia-telangiectasia mutated and RAD3-related (ATR), a serine/threonine protein kinase involved in the response to DNA stress, restored oxaliplatin sensitivity in a cellular model of oxaliplatin resistance. Combined application of the ATR inhibitor VE-822 and oxaliplatin resulted in strong synergistic effects in six different colorectal cancer cell lines and their oxaliplatin-resistant subclones, promoted DNA single- and double-strand break formation, growth arrest, and apoptosis. This treatment also increased replicative stress, cytoplasmic DNA, and signals related to immunogenic cell death such as calreticulin exposure and HMGB1 and ATP release. In a syngeneic colorectal cancer mouse model, combined administration of VE-822 and oxaliplatin significantly increased survival by promoting antitumor T-cell responses. Finally, a DNA repair gene signature discriminated sensitive from drug-resistant patients with colorectal cancer. Overall, our results highlight the potential of ATR inhibition combined with oxaliplatin to sensitize cells to chemotherapy as a therapeutic option for patients with colorectal cancer. Significance: These findings demonstrate that resistance to oxaliplatin in colorectal cancer cells can be overcome with inhibitors of ATR and that combined treatment with both agents exerts synergistic antitumor effects.