The effect of cesium (Cs) adsorption on the mesoscopic structure of the clay minerals vermiculite and weathered biotite (WB) in suspensions was elucidated by small-angle X-ray scattering (SAXS). The clay minerals form multilayered structures, and the Cs cations (Cs+) are strongly adsorbed in the interlayer space of the soil clays, in particular vermiculite and WB. SAXS was used to monitor the relationship between Cs+ adsorption at the clay interlayers and the structural changes at length scales from 1 to 1000 angstrom. The variation in the distance between the neighboring clay sheets and the spatial arrangement of the clay sheets with and without Cs+ were clarified. Our quantitative analyses revealed that the number of stacked layers of pure vermiculite was decreased by Cs+ addition, whereas that of WB increased. Moreover, the average distance between the neighboring layers of vermiculite in suspension was larger than that of WB, which reflects the different conditions of Cs+ intercalation. These findings provide fundamental insights that are important for predicting the environmental fate of radioactive Cs in contaminated regions and for developing methods for extracting Cs from soil.