In this study, 3-aminopropyltriethoxysilane (APTES) modified γ-alumina nanoparticles were utilized to improve the copper removal efficiency of polyethersulfone (PES) membranes. Alumina nanoparticles were first modified by APTES silane coupling agent before impregnating into PES composite membranes. The PES membranes were fabricated by incorporating three different amounts of modified nanoparticles by a phase inversion process. The prepared membranes were characterized using field emission scanning electron microscopy, Fourier transform infrared, X-ray diffraction, thermogravimetric analysis (TGA), dynamic mechanical analysis, water contact angle, water flux, and porosity measurements. The Cu(II) removal and adsorption capacity of the membranes were also analyzed. The addition of nanoparticles increased the thermal stability, hydrophilicity, total porosity, Brunauer–Emmett–Teller surface area, and glass transition temperature of the membranes. TGA confirmed a suitable uptake of the nanoparticles during the membrane fabrication process. The water permeation of the membranes also increased significantly. Membranes synthesized with 4 wt % nanoparticles showed the highest rejection for copper ions of 87%. Adsorption isotherms were tested using Langmuir and Freundlich models, where the Freundlich isotherm model resulted in the best fitting.