In spite of many efforts to grasp the nature of porous nanomaterials, it is hard to find research work addressing empirical evidence for selective water permeation through their channels or pores. Herein, we report the experimental proof of selective water permeation through cucurbit  uril (CB ) with a portal diameter of 3.9 angstrom along with quantum mechanics calculation results elucidating the mechanisms underlying the selective water transport. CB improved the water/salt permselectivity of CB -polyamide thin-film nanocomposite (CB-TFN) membranes since ion passage was inhibited by a high energy barrier imposed by the CB 's portals while the portals are energetically favorable from the perspective of water transport. This difference in water and salt's permeabilities stems from its carbonyl-fringed portals, which are cut out for size exclusion and negatively charged for charge repulsion. Due to the rigidity, CB-TFN membranes were found to be more resistant to compaction under elevated pressures. Such unique characteristics of CB allowed CB-TFN membranes to outperform newly developed TFN membranes as well as commercial RO membranes.