This study aimed to develop a novel radiosensitizer consisting of platinum nanoparticles (Pt NPs) as a high-atomic-number element in order to maximize the generation of ROS under ionizing radiation at the tumor site. Pt NPs were produced via a green and facile method in the presence of gelatin (Gel) as both reducing and stabilizing agent. After determining the physical structure and chemical composition of Pt@Gel NPs by STEM, FeSEM, EDS, DLS, XRD and FTIR, in vitro cytotoxicity on human umbilical vein endothelial cells (HUVEC) and breast cancer cell line (4T1) was evaluated by MTT assay. Finally, ROS generation assay, calcein AM/PI staining assay and clonogenic test were performed on 4T1 cells under X-Ray irradiation to evaluate the radioenhancment efficiency of Pt@Gel. The prepared NPs exhibited spherical and uniform shapes and narrowly distributed sizes in addition to an acceptable radiosensitization capability. The nanosystem provided higher levels of intracellular ROS in malignant cells and enhanced cancer cell death rate under X-Ray irradiation. Overall, the findings suggested that Pt@Gel could be a safe and effective alternative to existing radiosensitizers and potentially be employed for the treatment of breast cancer.