Radiotherapy is an important treatment for malignant tumors. However, tumor radioresistance remains the main factor limiting the efficacy of radiotherapy, which leads to tumor recurrence and metastasis. The intracellular DNA damage response pathway is activated in the presence of damage DNA. Studies have shown that the DNA damage response affects tumorigenesis and is closely associated with sensitivity to tumor radiotherapy, making it an extremely promising therapeutic target for clinical cancer treatment. Phase I clinical trials of small-molecule DNA damage response inhibitors are underway. In this article, the role of the DNA damage response in tumors, as well as the potential applications of key DNA damage response genes and repair pathways as biomarkers and therapeutic targets for radiosensitization therapy, were reviewed.