The　integration　of　high-nuclearity　lanthanide-oxo　(Ln-O)　clusters　into　polyoxotungstates　(POTs)　presents　a　formidable　synthetic　challenge　due　to　intricate　assembly　mechanisms　and　the　labile　nature　of　lanthanide　coordination.　Herein,　we　present　an　innovative　alkaline-mediated　templating　strategy　for　constructing　unprecedented　Ln-oxo-cluster-encapsulated　POTs　through　the　synergistic　assembly　of　lacunary　polyoxotungstate　precursors　with　in　situ　generated　Ln-oxo　clusters.　This　approach　employs　basic　N-containing　ligands　and　selenite　(SeO32−)　anions　as　synergistic　structure-directing　agents.　Guided　by　this　approach,　we　have　successfully　synthesized　two　series　of　giant　Ln21-oxo-cluster-embedded　polyoxotungstates,　H23K30(H2O)18{[Ln21(H2O)9(WO6)(SeO3)6][Ln3(SeO3)2]2(GeW10O38)9}·solvent　(1-Ln)　(Ln　=　Eu,　Gd,　Tb,　Dy)　and　H27Na3K21(H2O)11(MePIP)2{[Ln21(H2O)9(WO6)(SeO3)6][Ln3(SeO3)2]2(GeW10O38)9}·solvent　(2-Ln)　(Ln　=　Eu,　Tb,　MePIP　=　N-methylpiperazine).　Notably,　these　compounds　feature　a　rare　pinwheel-shaped　high-nuclearity　Ln21-oxo-cluster-embedded　polyanion,　{[Ln21(H2O)9(WO6)(SeO3)6][Ln3(SeO3)2]2(GeW10O38)9}　({Ln27Se10Ge9W91}),　with　a　three-layered　sandwich　structure.　Compounds　1-Eu　and　2-Eu　exhibit　distinct　proton　conduction　properties　due　to　their　different　packing　structures.　In　particular,　1-Eu　demonstrates　a　high　single-crystal　proton　conductivity　of　1.8　×　10−2　S　cm−1　at　98%　RH　and　25　°C　along　the　[001]　channel　direction,　outperforming　most　crystalline　porous　materials.　Additionally,　1-Eu　exhibits　temperature-dependent　fluorescence.　The　1-Eu0.25Dy0.75　sample　can　emit　macroscopic　white　light　under　blue　irradiation.　This　work　provides　a　feasible　strategy　for　constructing　high-nuclearity　Ln-oxo-cluster-containing　POTs　and　expands　their　application　in　smart　proton　conductive　materials　and　multicolor　luminescent　systems.　©　2025　The　Royal　Society　of　Chemistry.