Metal–organic frameworks (MOFs) have been extensively studied as promising nanofillers in developing high-performance polymer nanocomposite membranes (PNMs) for efficient water/ion separation applications. However, given the ambiguous role of embedded MOFs, achieving simultaneous improvement in both water permeability and water/ion selectivity of PNMs remains challenging. Here, we elucidates fundamental water and ion transport properties of MOF/PNMs to better understand the role of embedded MOFs in polymer matrices. We prepared freestanding PNMs consisting of a cross-linked poly(ethylene glycol) (XPEG)-based hydrogel and nanoporous zeolitic imidazole framework-8 (ZIF-8) exhibiting high diffusivity selectivity. The transport studies and material characterizations, especially with Raman mapping analysis showing a homogeneous distribution of permeating water molecules throughout ZIF-8/XPEG PNM, revealed that the incorporated ZIF-8 acts as an additional water-permselective channel inside the polymeric matrix, which leads to an unusual “reverse-selective” ion transport behavior. Ultimately, 20 wt % of ZIF-8 loading could significantly enhance both water permeability (∼240%) and water/NaCl selectivity (∼160%) compared to a pure polymer membrane by overcoming the conventional permeability–selectivity trade-off limitation. Our finding provides new insights for developing advanced PNMs for water/ion separation.