放射治疗（英文：Radiation therapy；简称放疗，香港称为电疗，与物理治疗中的电疗(electrotherapy)相异）是使用辐射作为治疗疾病的方式。其原理是大量的辐射所产生的能量可破坏细胞的染色体，使细胞停止生长，从而消灭可快速分裂和生长的癌细胞。放射治疗最常作为直接或辅助治疗癌症的方式。此外在骨髓移植前，也必须用辐射照射全身，以消除所有恶性癌或HIV细胞。其他疾病如三叉神经痛也可利用辐射来治疗。治疗轻微疾病较少使用电疗，主要担心辐射影响人体健康。所以除非是大病例如癌症，否则一般不会使用。在治疗恶性肿瘤通常会结合手术、化疗和电疗一起进行。但确切的配合模式需要由医生进行评估判断。

Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is therapy using ionizing radiation, generally as part of cancer treatment to control or kill malignant cells. Radiation therapy may be curative in a number of types of cancer if they are localized to one area of the body. It may also be used as part of adjuvant therapy, to prevent tumor recurrence after surgery to remove a primary malignant tumor (for example, early stages of breast cancer). Radiation therapy is synergistic with chemotherapy, and has been used before, during, and after chemotherapy in susceptible cancers. The subspecialty of oncology that focuses on radiotherapy is called radiation oncology.