<p>Highly permeable and selective biomimetic membranes that can feel and recognize valuable ion species have attracted enthusiastic interest due to their analogous behavior with biological ion channels and potential applications in rigorous ion sieving. However, designing and developing single-species selective membranes that can isolate monovalent cations such as K⁺/Na⁺ remains a tremendous challenge due to the sub-nanometer ion size, as well as the angstrom-sized difference. Considering the non-homogeneous heterostructure of KcsA channels and -COOH groups generally showing lower K⁺ affinity, we propose the 1D MOF (rich in -COOH groups)-in-2D COF concept, aiming to enhance K⁺/Na⁺ separation through strategic construction of heterogeneous ion transport channels, therefore narrowing the pore size of pristine COF membrane, and weakening the K⁺-channel wall interactions. Concretely, by interlocking and in situ immobilized growth, the pristine COF membrane is capable of capturing MOF ligands and metal ions in sequence to form 1D MOF-in-2D COF hetero-structured composite membranes. Benefiting from the molecular-level interlinked hybridization of covalent and metal organic hetero-frameworks induced by the coordination interaction between the -NH groups in COFs and the Cu centers from MOFs, the composite membrane enables rapid diffusion of K⁺ in confined heterogeneous channels, thus leading to unprecedented cation sieving performance with K⁺/Na⁺ selectivity approaching 10² and K⁺/Mg²⁺ selectivity exceeding 10³. This membrane design concept exploits a viable avenue for developing single-species selective biomimetic membranes to achieve ultrahigh separation performance.</p>