Water pollution caused by the discharge of wastewater containing organic dyes, particularly anionic dyes, is one of the major environmental challenges. In this study, mesoporous silica SBA-15 was first synthesized via the sol–gel method, followed by the fabrication of a magnetic Fe2O3/SBA-15/PRh nanocomposite through in situ polymerization. The synthesized nanocomposite was evaluated as an efficient adsorbent for the removal of Acid Blue 25 (AB25) from aqueous solutions. The structural and physicochemical properties of the synthesized adsorbent were characterized using FT-IR, XRD, FESEM, TEM, BET, VSM, and TGA analyses. The effects of solution pH, contact time, adsorbent dosage, initial dye concentration, and temperature on the adsorption process, as well as the kinetic and thermodynamic behaviors of the system, were investigated. The characterization results confirmed the successful synthesis of the nanocomposite while preserving the mesoporous structure of SBA-15 after modification. The optimum adsorption conditions were pH 2, an adsorbent dosage of 0.03 g, and a contact time of 90 min. The equilibrium data were best described by the Langmuir isotherm model, with a maximum adsorption capacity of 833.33 mg g⁻¹. Furthermore, the enthalpy (ΔH°) and entropy (ΔS°) values were calculated to be −29.706 kJ mol⁻¹ and −88.436 J mol⁻¹ K⁻¹, respectively, while the negative ΔG° values at all investigated temperatures confirmed the spontaneous nature of the adsorption process. Overall, the Fe2O3/SBA-15/PRh nanocomposite can be considered an effective adsorbent for the removal of anionic dyes from aqueous media.