Potassium cobalt-hexacyanoferrate (KCoFC)-impregnated fibers were prepared by radiation-induced cografting of vinylbenzyltrimethylammonium chloride (VBTAC) and N-vinyl-2-pyrrolidone (NVP) and subsequent chemical modifications. A 10 mg-Cs/L cesium chloride solution with various sodium chloride concentrations up to 0.5 M was forced to flow through the KCoFC-fiber-packed bed to acquire a breakthrough curve. The dynamic binding capacity of the bed for cesium with 0.5 M sodium chloride was 30-fold higher than that without sodium chloride. The adsorption isotherm of the KCoFC fiber for cesium ions in seawater was correlated using the Langmuir equation with a saturation capacity of 20 mg-Cs/g-dry.