Abstract: Radiotherapy is one of the important means of combined therapy for treating tumors. However, radioresistance is a serious issue that affects the curative effect and prognosis of radiotherapy for tumor patients. Given the complex mechanism of radioresistance in tumor cells, the specific switch molecules that can regulate radiosensitivity are not yet discovered. Circular RNA (circRNA) is a kind of closed circular RNA molecules that covalently bind the 3'-end and 5'-end by trans-splicing with high abundance, stable structure, and strong specificity. CircRNA is involved in tumorigenesis, development, invasion, and metastasis and can be used as a novel tumor molecular marker and potential therapeutic target. In addition, circRNA is differentially expressed in irradiated tumor cells and can serve as a sponge to regulate microRNA and its downstream signaling pathways that are related to tumor radioresistance. Therefore, the research on circRNA might be a promising breakthrough toward overcoming tumor radioresistance. In this study, we reviewed the progress in the research on circRNAs as novel tumor markers, as well as the research prospects regarding the application of radiotherapy.