We have revealed the fundamental mechanism of specific Cs+ adsorption into Prussian blue (PB) in order to develop high-performance PB-based Cs+ adsorbents in the wake of the Fukushima nuclear accident. We compared two types of PB nanoparticles with formulae of FeIII4[FeII(CN)6]3·xH2O (x = 10–15) (PB-1) and (NH4)0.70FeIII1.10[FeII(CN)6]·1.7H2O (PB-2) with respect to the Cs+ adsorption ability. The synthesised PB-1, by a common stoichiometric aqueous reaction between 4Fe3+ and 3[FeII(CN)6]4−, showed much more efficient Cs+ adsorption ability than did the commercially available PB-2. A high value of the number of waters of crystallization, x, of PB-1 was caused by a lot of defect sites (vacant sites) of [FeII(CN)6]4− moieties that were filled with coordination and crystallization water molecules. Hydrated Cs+ ions were preferably adsorbed via the hydrophilic defect sites and accompanied by proton-elimination from the coordination water. The low number of hydrophilic sites of PB-2 was responsible for its insufficient Cs+ adsorption ability.