The development of simple techniques for the separation and purification of recombinant proteins plays an important role in many of the advancements made in biotechnology and nanotechnology. Herein, we report an easy method for the efficient purification of polyhistidine affinity-tagged (His-tagged) proteins by using Ni²⁺-decorated superparamagnetic particles. Monodisperse Ni_0.3Fe_0.7Fe₂O₄ nanoparticles were prepared via a facile and economical one-pot hydrothermal process. Owing to the characteristic molecular recognition ability between nickel(Ⅱ) ions and the polyhistidine affinity tag, the nanoparticles could be successfully employed to selectively bind and separate His-tagged cyan fluorescent protein (CFP) from an E. coli cell lysate in a recyclable process. Moreover, by changing the divalent metal precursors, various other metal-decorated magnetic nanoparticles can be obtained. This approach offers the possibility of constructing metal-decorated nanoparticles through a simple method and will be highly beneficial in further applications of nanoparticle-based technologies.