Recent studies have emphasized that the surface properties of support layers in thin-film composite (TFC) membranes play a significant role in reverse osmosis (RO) performance. In this study, we used carbon nanotubes (CNTs) to tune the surface properties of microporous polysulfone membranes. The prepared CNT/PSf nanocomposite supports showed significantly improved surface porosity while maintaining both surface pore radius and hydrophobicity. Such surface characteristics resulted in the defect-free formation of a polyamide (PA) selective layer possessing a large surface area, which led to enhancement in the flux of PA-TFC membranes. PA-TFC membranes prepared with CNT/PSf nanocomposite supports showed improved water permeance up to 35% without losing salt rejection compared to the bare PA-TFC membranes. The results revealed that the surface porosity of the support is a dominant factor influencing the water permeance of TFC membranes rather than the pure water flux inside the support itself or the thickness of the PA layer. We also propose a direction for generating optimal supports through a comparison study between CNTs and hydrophilic pore formers as additives in the support material.