The spinel ferrite CoFe_1.98Sm_0.02O₄, prepared by sol–gel autocombustion technique using maleic acid as combustion agent, was successfully functionalized with chitosan (CS) and epichlorohydrine (ECH). The insertion of the samarium cations into the spinel matrix was proved by X-ray diffraction and Fourier transform infrared analyses. Likewise, the specific magnetization value indicated supplementary evidence of the successful adaptation of rare-earth cations into the spinel structure. The functionalization with CS and the cross-linking with ECH were demonstrated by infrared spectroscopy. The developed material (CoFe_1.98Sm_0.02O₄@CS-ECH) was applied as sorbent for the removal of anionic dye (Orange II) in the batch mode. Adsorption process was investigated by kinetics, equilibrium, and thermodynamic studies. The model-based optimization of the adsorption process was performed according to the response surface methodology to evaluate the influence of pH and sorbent dosage. The optimization yielded the maximum value of color removal efficiency of 96.31% using only 1.60 g/L adsorbent dosage at pH 5.2.