The adsorption sites of Cs on montmorillonite clays were investigated by theoretical Cs-133 chemical shift calculations, Cs-133 magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and X-ray diffraction under controlled relative humidity. The theoretical calculations were carried out for structures with three stacking variations in the clay layers, where hexagonal cavities formed with Si-O bonds in the tetrahedral layers were aligned as monoclinic, parallel, alternated; with various d-spacings. After structural optimization, all Cs atoms were positioned around the center of hexagonal cavities in the upper or lower tetrahedral sheets. The calculated Cs-133 chemical shifts were highly sensitive to the tetrahedral Al (Al-T)-Cs distance and d-spacing, rather than to the Cs coordination number. Accordingly, three peaks observed in our theoretical spectra were interpreted to be adsorbed Cs around the center of hexagonal cavity with or without Al-T and on the surface in the open nanospace. In a series of Cs-133 MAS NMR spectral changes for partial Cs substituted samples, the Cs atoms are preferentially adsorbed at sites near Al-T for low Cs substituted montmorillonites. The presence of nonhydrated Cs was also confirmed in partially Cs substituted samples, even after being hydrated under high relative humidity.