Polyethyleneimine (PEI) has been reported to have good potential for the adsorption of metal ions. In this work, PEI was covalently bound to NiFe2O4@SiO2 nanoparticles to form the new adsorbent NiFe2O4@SiO2–PEI. The material allowed magnetic separation and was characterized via powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform IR spectroscopy (FT-IR), and thermogravimetry-differential thermal analysis (TGA-DTA). The adsorption of CrO42‒, Ni2+, and Pb2+ ions from aqueous solutions was studied at different pH, temperatures, metal ion concentration, and adsorbent dosage. Maximum adsorption capacities of 149.23, 156.68, and 161.25 mg/g were obtained for CrO42‒, Ni2+, and Pb2+, respectively under optimum conditions using 0.075 g of the adsorbent material 250 mg/L ion concentration, at pH = 6.5 and room temperature.