Modification of polysulfone (PSf) membranes with cellulose nanomaterials (CNMs) including cellulose nanocrystals (CNC) and cellulose nanofibers (CNF) to prepare PSf-CNM nanocomposite mixed-matrix membranes (PSf-CNM NMMMs) with enhanced properties was investigated. Results pointed to slight effect of CNM type on pure water flux (PWF) of the prepared PSf-CNM NMMMs when PSf concentration was selected to be 20 wt.%. Additionally, it was demonstrated that the contribution of CNC to the morphology and performance of the PSf-CNC NMMMs is influenced by parameters such as membrane preparation technique (immersion precipitation (IP) and vapor-induced phase separation (VIPS)), PSf concentration in the casting solution (20 and 30 wt.%) and the casting thickness (50, 100 and 200 µm). It was also found that when PSf concentration was 20 wt.%, pure water flux (PWF) of the prepared PSf-CNM NMMMs was severely affected by the membrane preparation technique. Furthermore, irrespective of the membrane preparation technique, the highest PWF was reached at optimum CNC content of 0.5%. It was found that for PSf concentrations of 20 and 30 wt.%, CNC content of 0.5% enhances the membrane PWF by 327.4% and 256%, respectively. This is due to the hydrophilic nature of CNC. At PSf concentration of 30 wt.%, the contribution of CNC is more pronounced at higher casting thicknesses. The highest increases in PWF of PSf-CNC NMMMs for casting thicknesses of 50, 100 and 200 µm were found to be 171.3%, 610.2% and 457.3%, respectively. These were achieved at CNC content of 0.5%, 1% and 1%, respectively. In case of purification of methylene blue (MB) wastewater, it was concluded irrespective of the membrane preparation technique, CNC content positively affects dye removal efficiency. Finally it was concluded that the PSf-CNC NMMM prepared under conditions of PSf concentration of 30 wt.%, CNC content of 0.5% and casting thickness of 50 µm, possesses the highest PWF of 22.88 L/(m 2 .h) and MB rejection efficiency of 98.1%.