Monovalent cation permselective membranes (MCPMs) are highly desirable for the extraction of Li⁺ and Na⁺ ions from earth-abundant sources, such as salt lakes and seawater. Metal–organic frameworks (MOFs) are promising functional nanomaterials with excellent potential for ion separation technologies owing to their regular structure and tunable pore sizes. However, the successful use of MOFs in ion separation membranes is still challenging owing to the numerous difficulties in preparing ultrathin and defect-free MOF membranes. Here, we proposed a facile post-synthetic method for the preparation of UiO-66(Zr/Ti)-NH₂ and subsequently immobilized UiO-66(Zr/Ti)-NH₂ in an ultrathin polyamide layer (~100 nm). The resulting thin-film nanocomposite membranes presented high monovalent cation permeation and excellent selectivity for mono-/di-valent cations. The P_{Na⁺/Mg²⁺} and P_{Li⁺/Mg²⁺} permselectivities of the best-performing thin-film nanocomposite membrane were 13.44 and 11.38, respectively, which were 3.8 and 5.1 times higher, respectively, than those of the commercial state-of-art CSO membrane.