Radiotherapy is a critical treatment strategy for breast cancer. However, its wide application is sometimes restricted by radioresistance and radiotoxicity. Trametes robiniophila Murr. (Huaier), an officinal fungus used as a traditional Chinese medicine (TCM), is reported to have multi-biological functions during cancer treatment. Yet, its radiosensitization effects have not been evaluated to date. In the present study, using HTA 2.0 transcriptome microarray assay, Huaier was found to downregulate genes related to the cell cycle, cell division, cell cycle phases and DNA repair. This investigation utilized a colony formation assay to confirm the ability of Huaier to sensitize breast cancer cells to radiotherapy. Flow cytometry, immunofluorescence staining and western blotting were used to illustrate the sensitization mechanism. Our findings suggest that Huaier causes G0/G1 arrest through downregulation of cell cycle-regulating proteins in MCF-7 and MDA-MB-468 cells, prolongs the persistence of γ-H2Ax foci after radiotherapy and interferes with the homologous recombination (HR) pathway of DNA repair by downregulating RAD51. These results suggest that Huaier has the ability to sensitize breast cancer cells to radiotherapy through regulation of the cell cycle and DNA repair pathway. Thus, Huaier may be a promising radiosensitizer for the treatment of breast cancer.