Magnetic Prussian blue/Fe₃O₄ core/shell nanoparticles were successfully fabricated via a facile one-pot method. The results of transmission electron microscope (TEM), X-ray diffraction patterns (XRD) and Fourier transform infrared spectra (FT-IR) reveal that the as-prepared products are 20–40 nm in diameter and have obvious core/shell structures, which consist of face-centered-cubic lattice Prussian blue and Fe₃O₄. They can be well dispersed in water and show obvious magnetism, which means they can be rapidly separated by a magnet. The results of adsorption experiments indicate that their maximal adsorption capacities for Cs⁺ are 145.8 and 132.6 mg g⁻¹, respectively. After magnetic separation, the Cs⁺ removal efficiency remains at more than 90% when the Cs⁺ concentration in solution is 50 μg mL⁻¹. The magnetization and Cs⁺ adsorption properties can be easily adjusted by changing the content of Fe₃O₄ in nanocomposites. The synthesis strategy and mechanism are also generally discussed. The resulting magnetic Prussian blue nanoparticles can be widely applied to treat radioactive waste water containing cesium ions.