This paper focused on the synthesis of Fe₃O₄@NiO core–shell magnetic nanoparticles for the highly efficient removal of Alizarin red S dye from contaminated effluent. The physicochemical properties of as-synthesized nanoparticles were characterized by X-ray diffraction, scanning electron microscopy, vibrating sample magnetometer, transmission electron microscopy, N₂ adsorption–desorption, and Fourier transform infrared spectroscopy instrumental analysis. Many adsorption parameters, i.e., contact time, adsorbent dosage, pH solution, initial Alizarin red S concentration, and process temperature, were applied to interpret the dye adsorption mechanism. Also, the reuse efficiency of Fe₃O₄@NiO in six cycles of adsorption–desorption was investigated. By applying many isotherms and kinetic models of adsorption, it was concluded that Alizarin red S removal using Fe₃O₄@NiO followed pseudo-second-order kinetic equation (R² = 0.9985) and Freundlich isotherm model (R² = 0.9947) with a maximum adsorption capacity of 223.30 mg/g. The effect of coexisting ions (bicarbonate, phosphate, sulfate, nitrate ions) on the removal of Alizarin red S was investigated. The results indicated that adsorption capacity has decreased by increasing the concentration of co-anions in the solution. The affinity sequence for the anion adsorption on the adsorbent is \({\text{NO}}_{{3}}^{ - }\) > \({\text{HCO}}_{{3}}^{ - }\) > \({\text{PO}}_{{4}}^{{{3} - }}\) > \({\text{SO}}_{{4}}^{2 - }\). Overall, as-synthesized Fe₃O₄@NiO core–shell magnetic nanoparticles indicated an efficient performance in dye adsorption, specifically Alizarin red S dye from contaminated wastewater, which may be useful for practical applications in large quantities.