In this study, surface-immobilized thiol-functionalized nanosilica poly(ether sulfone) (PES) membranes were prepared to improve the membrane selectivity and wettability during the filtration of heavy-metal solutions. The PES membranes were prepared by the phase inversion method to create more open and interconnected nanoporous structures. Nanosilica particles were functionalized with 3-mercaptopropyltrimethoxysilane (MPTMS) to initiate free thiol (-SH) groups on the surface and were used as the dispersed phase in ethanol. To increase the adsorption of nanoparticles on the membrane surface, the surface of the PES membranes was treated with Ar/O-2 plasma before being immersed in the dispersion phase. The cross-sectional morphology and surface composition of the membranes were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. Contact angle (CA) measurements confirmed that plasma treatment improves the wettability of PES membranes. Functionalized nanosilica was immobilized on the plasma-treated PES membrane, and its characterization and performance were investigated. The results showed that membrane rejection and selectivity were improved by the immobilization of thiol-functionalized nanosilica (MNS) on the surface of PES membranes without any significant reduction in flux properties.