Prussian blue (PB) has been well known as a pigment crystal to selectively sequestrate the radioactive cesium ion released from aqueous solutions owing to PB cage size similar to the cesium ion. Because the small size of PB is hard to deal with, the adsorbents containing PB have been prepared in the form of composites causing low sequestration efficiency of cesium. In this study, securely anchored PB nanocrystals on the surface of millimeter-sized porous polyacrylamide (PAAm) spheres (PB@PAAm) have been prepared by the crystallization of PB on the Fe adsorbed PAAm. The securely anchored PB nanocrystals have been demonstrated to be selective and efficient adsorbents for sequestration of the radioactive cesium. The well-interconnected-spherical pores and millimeter-sized diameter of the PB@PAAm adsorbents facilitated permeation of Cs into the adsorbent and ease of handling respectively. Especially the well-interconnected-spherical pores allowed that PB@PAAm showed 90% of its maximum Cs adsorption capacity within 30 min. The PB@PAAm showed an outstanding Cs capture ability of 374 mg/g, high removal efficiency of 85% even at low concentration of Cs (10 ng/L), and superior selectivity of Cs against interference ions of Na, K, Mg, and Ca.