Provided herein is a cesium adsorbent including: a support modified to have a carboxyl group on a surface thereof; and Prussian blue synthesized on the surface of the modified support, wherein the Prussian blue is at least partially chemically bound with the surface of the support. The cesium adsorbent may effectively adsorb cesium, which is a radioactive element released into the water and may be easily prepared using a simple solution process.