The report focuses on the preparation of chitosan mediated gold nanoparticles (Au@CS NPs) and theirapplication in radiotherapy. It highlights the use of gold nanoparticles as a way to enhance the efficacy of radiation therapy in cancer treatment. The process of fabrication of these gold nanoparticles using CS as both a stabilizing and reducing agent is detailed. CS, a biopolymer derived from chitin, is considered biocompatible and biodegradable and has been used largely in biomedicine. The properties of gold nanoparticles stabilized by CS were analyzed using various techniques such as FESEM, STEM, DLS, zeta potential, and UV–Vis. Finally, the in vitro assays were conducted to study the radioenhancement effect of Au@CS NPs on breast cancer cell line (4T1) as a radiosensitizer agent through the cell viability and reactive oxygen species (ROS) generation assays. The results show that Au@CS NPs could produce ROS within cancer cells and enhance sensitization of cancer cells against X‐ray.