The present invention relates to a radioactive cesium adsorbent, a method for producing the same, and a method for decontaminating the environment from radioactive cesium with the adsorbent. The radioactive cesium adsorbent of the present invention includes a hydrophilic fiber substrate supporting a Prussian blue analogue, in particular, Prussian blue, and the Prussian blue analogue is immobilized in the inside of the fibers.