In this study, the magnetic SBA-15/Poly-L-Lysine nanocomposite was synthesized and characterized as a novel adsorbent for the removal of Acid Blue 25 (AB25) from aqueous solutions. The structure and properties of the nanocomposite were investigated using XRD, BET, FTIR, VSM, FESEM, TEM, and TGA analyses, and the results confirmed the successful formation of the nanocomposite and its desirable structural, textural, morphological, magnetic, and thermal properties. The effects of the parameters affecting the adsorption process were investigated, and the optimum conditions were determined as a pH of 2, a contact time of 60 min, a temperature of 25 °C, and an adsorbent dosage of 0.02 g. Under these conditions, the experimental adsorption capacity of AB25 was obtained as 261.36 mg/g. The kinetic study showed that the pseudo-second-order model provided a better fit to the experimental data. Furthermore, the values of ΔH∘=−0.07845 kJ/mol and ΔS∘=-19.59 J/(mol.K) indicated the exothermic nature of the adsorption process and a decrease in disorder at the adsorbent–solution interface, respectively. Overall, the results demonstrated that Mag-SBA-15/PLL has suitable potential for the removal of AB25 from aqueous environments.