Abstract: Metformin (C4H11N5) is one of the main drugs used to treat type 2 diabetes. Research evidence shows that metformin can inhibit tumor growth and improve the prognosis of patients with malignant tumors. In recent years, an increasing number of clinical and basic research reports have indicated that metformin can improve the radiosensitivity of various tumor cells. However, the specific mechanism by which metformin enhances the radiosensitivity of malignant tumor cells has not yet been fully elucidated. This review discusses the latest discoveries related to the application of metformin in combination with radiotherapy and highlights its efficacy and underlying molecular mechanisms in different tumors, thereby providing a theoretical basis for the use of metformin in clinical treatment of malignant tumors.