We examined the highly selective Cs adsorption ability of a potassium copper hexacyanoferrate (KCuHCF) slurry ink in seawater, i.e. very low Cs concentration in the presence of Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl⁻ ions at high concentrations. The rational synthesis of the KCuHCF nanoparticle (KCuHCF-NPs, A_yCu[Fe(CN)₆]_1−x·zH₂O) slurry ink was carried out using a Y-type micro-mixer and the composition dependence of the Cs-adsorption behavior was investigated. When a solution-to-adsorbent ratio of 10 000 mL g⁻¹ was applied, batch experiments revealed that the Cs adsorption equilibrium was achieved within 10 min, with removal efficiency reaching as high as 99% of the Cs present (initial concentration is 10 μg L⁻¹) and a distribution coefficient (K_d) as high as 10⁶, even in the presence of competing alkali metal cations in seawater at high concentrations. For practical use, large flocs of the KCuHCF-NPs could be immediately coagulated after Cs adsorption using a high molecular-weight ionic polymer as a coagulant, along with regular Fe²⁺ inorganic flocculants at the pH of seawater.