This study investigates the effect of multiwalled carbon nanotubes (MWCNTs) on the performance of polyethersulfone membranes in cerium ion removal from aqueous solution. Neat polyethersulfone (PES) membrane and mixed-matrix membranes (MMMs) containing MWCNTs are fabricated by the wet phase inversion technique. The MWCNTs are functionalized by (3-aminopropyl)-trimethoxysilane (APTMS) to improve their adhesion to the polymeric matrix. The MWCNTs (before and after functionalization) are analyzed using Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and thermogravimetric analysis (TGA). Morphological properties of the fabricated membranes are evaluated by scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM) methods. Fabricated membranes are subjected to further experiments in order to evaluate cerium ion removal efficiency. Permeation test results show that permeate flux and cerium (Ce) ion rejection increase as MWCNT content increases in the polymer matrix. The results also show that permeate flux and cerium ion rejection increase as operating pressure and feed flow rate increase while inversely decreased by increasing the cerium ion concentration in the feed solution. Application of ethylenediaminetetraacetic acid disodium (EDTA) drastically increases cerium ion rejection.