We report on the synthesis of new Dion–Jacobson phase three-layer perovskite CsBa2Ta3O10 crystals by conventional solid-state reaction and fabrication of their nitrided Ba2Ta3O10 nanosheets via a nitridation–protonation–intercalation–exfoliation route for visible-light-driven photocatalytic water splitting application. The two-dimensional crystal structure of CsBa2Ta3O10 refined by the Rietveld refinement method belongs to the simple tetragonal system and consists of corner-shared distorted TaO6 octahedra and Ba cations in cuboctahedral sites forming triple-layer perovskite slabs. The valence and conduction band edge potentials estimated by density functional theory with respect to vacuum scale and normal hydrogen electrode indicate that CsBa2Ta3O10 is a UV-light-active semiconductor that can be used for photocatalytic water splitting and decomposition of organic pollutants. The projected electronic wave functions contour plot of CsBa2Ta3O10 in the (1 0 0) plane exhibited two different Ta–O bonds. The nit...