<p>Perovskite oxides are promising for Cs⁺ separation owing to their rich composition, structural tunability, and chemical stability. However, the difficulty lies in clarifying the composition-structure-function relationship to direct the materials design. Herein, Dion-Jacobson layered perovskite oxides (D-J-LPOs) ACa_2−<i>x</i>La_<i>x</i>Nb_3−<i>x</i>Ti_<i>x</i>O₁₀·<i>n</i>H₂O (<i>A</i> = K⁺, Rb⁺, Cs⁺, H⁺; <i>x</i> = 0–3) are investigated to elucidate the influence of <i>A</i>-site (K⁺, Rb⁺, Cs⁺, H⁺) and <i>B</i>-site (Nb⁵⁺/Ti⁴⁺) cations on Cs⁺ adsorption. Only HCa_2−<i>x</i>La_<i>x</i>Nb_3−<i>x</i>Ti_<i>x</i>O₁₀·<i>n</i>H₂O with the suitable interlayer distances and the presence of hydrated protons/interlayer water molecules can effectively capture Cs⁺ through ion exchange. Even under competing ions or in actual environmental water, HCa_2−<i>x</i>La_<i>x</i>Nb_3−<i>x</i>Ti_<i>x</i>O₁₀·<i>n</i>H₂O can selectively capture Cs⁺. Importantly, as the <i>B</i>-site Nb⁵⁺/Ti⁴⁺ ratio increases, the Cs⁺ adsorption performances are markedly enhanced. Due to the higher electronegativity of Nb⁵⁺, with the increase of the Nb⁵⁺/Ti⁴⁺ ratio, the interlayer protons are gradually attached from [TiO₆] to [NbO₆], leading to the increase of Bronsted acidity of the material and charge density of the anionic layer, facilitating the Cs⁺ ion exchange. This study clarifies that the <i>A</i>-site cation in D-J-LPOs governs the occurrence of Cs⁺ ion exchange, while the <i>B</i>-site cation modulates the efficiency. This work pioneers insights into the composition-structure-function relationship for designing advanced materials for radionuclide remediation.</p>