Highly stable silver nanoparticles were successfully synthesized by gamma ray irradiation in the presence of sodium alginate. The silver nanoparticles were characterized by UV–vis spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Their particle sizes were in the range of 6–30 nm. The as-obtained Ag nanoparticle dispersion was stable for over 6 months at room temperature.