Availability and access to clean water have become a global issue of concern as human populations grow, industrial and agricultural activities expand. Both desalination and reuse wastewater represent a major opportunity to reduce water scarcity. Seawater desalination by reverse osmosis (RO) is a costly process and requires reliable pre-treatment techniques such as ultrafiltration (UF) processes to make the process much more economically attractive. In view of this, the main objective of this study is to fabricate and characterize low-biofouling UF nanocomposite membranes by incorporating different weight percentages of silver nanoparticles (AgNPs) loadings (0.5, 1.0, 2.0 wt.%) by in situ blending method for seawater desalination pre-treatment and surface water treatment applications. The nanocomposite membranes were fabricated via phase inversion method using a dope solution containing 15 wt.% polyethersulfone (PES) as the base polymer dissolved in 82 mL of 1-Methyl-2-pyrrolidinone (NMP). All membrane types were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), fourier transform infrared radiations spectroscopy (FTIR), x-ray diffraction (X-RD), water contact angle (WCA), membranes porosity (ɛ %), mechanical strength and inductively coupled plasma-optical emission spectroscopy (ICP-OES) were studied as well. Membrane performance was tested using a dead-end unit to evaluate the effect of AgNPs on membrane pure water flux and filtration performance using two model foulants humic acid (HA) and bovine serum albumin (BSA). The antifouling property of the membrane was analyzed by calculating the fouling resistance parameters of BSA. The antibacterial activity of membranes was tested using two types of bacteria which are Bacillus subtilis (B. subtilis) and Escherichia coli (E. coli). The results showed that the WCA was reduced from 68˚ for the unmodified PES membrane to 58˚ for PES-0.5 AgNPs membrane. The PES-0.5 AgNPs membrane displayed improved water flux by 65.55% and both HA and BSA rejections were 90.04% and 82.41% respectively. Although the WCA of the membrane was further reduced when a higher AgNPs amount (1.0 and 2.0 wt.%) was used, the results of membranes suffered from decreased ultimate strength and significant nanoparticles leaching. A high flux BSA recovery of about 97.29% was achieved for the modified membrane with 2.0% AgNPs compared to 71.12% for the neat membrane. In terms of the antibacterial performance evaluation, modified PES membranes showed the excellent property in biofouling mitigation and E. coli results show more sensitive to AgNPs than B. subtilis due to their thinner cell walls structure compared to B. subtilis.