To solve the traditional radiotherapy obstacles, and also to enhance the radiation therapy efficacy various radiosensitizers have been developed. Radiosensitizers are promising agents that under X-ray irradiation enhance injury to tumor tissue by accelerating DNA damage. In this report, silver-silver sulfide nanoparticles (Ag-Ag2S NPs) were synthesized via a facile, one-pot and environmentally friendly biomineralization method. Ag-Ag2S was coated with bovine serum albumin (BSA) in situ and applied as an X-ray sensitizer to enhance the efficiency of radiotherapy. Also, folic acid (FA) was conjugated to Ag-Ag2S@BSA to impart active targeting capability to the final formulation (Ag-Ag2S@BSA-FA). Prepared NPs were characterized by transmission electron microscopes (TEM), scanning electron microscope (SEM), dynamic light scattering (DLS), ultraviolet–visible spectroscopy (UV–Vis), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. Results show that most of the NPs have well-defined uniform Janus structures. The biocompatibility of the NPs was then evaluated both in vitro and in vivo. A series of in vitro assays were performed on 4T1 cancer cells to evaluate the therapeutic efficacy of the designed NPs. In addition, the radio-enhancing ability of the NPs was tested on the 4T1 breast cancer murine model. MTT, live and dead cell staining, apoptosis, ROS generation, and clonogenic in vitro assays demonstrated the efficacy of NPs as radiosensitizers in radiotherapy. In vivo results as well as H&E staining tumor tissues confirmed tumor destruction in the group that received Ag-Ag2S@BSA-FA NPs and exposed to X-ray. The results showed that prepared tumor-targeted Ag-Ag2S@BSA-FA NPs could be potential candidates as radiosensitizers for enhanced radiotherapy.