Radiotherapy still fails to achieve satisfactory efficacy in the treatment of malignant tumors, and applying radiotherapy sensitizers is an effective method to improve the efficacy of radiotherapy. Gold nanomaterials can effectively increase the sensitivity of tumor cells to radiotherapy due to their high atomic numbers. Gold nanoclusters have more excellent radiobiological and radiophysical properties due to their smaller size. This paper reviews the special radiobiological and radiophysical properties of gold nanoclusters and describes in detail their sensitizing effects in external radiation radiotherapy, radionuclide therapy, and X-ray induced photodynamic therapy.