A new approach to an efficient and selective extraction of Cs⁺ ions from water, sea water enriched with Cs⁺ and a radioactive solution simulating the effluents of the Fukushima reactors (¹³⁷Cs, 29 kBq L⁻¹) was developed by using porous silica- or glass-based nanocomposites containing Prussian blue type nanoparticles, Co²⁺/[Fe(CN)₆]³⁻, with sizes below 10 nm. A particular emphasis is given on the kinetics of cesium sorption fitted by using the classical reaction order model as well as a diffusion model in order to better understand the sorption mechanism. Compared to the amount of Co²⁺/[Fe(CN)₆]³⁻ nanoparticles, the sorption capacities of studied nanocomposites are more than three times higher than the ones observed for the respective bulk materials. These nanocomposites present a high selectivity to Cs⁺ and extract it in trace amounts.