Industrial activities that utilize nuclear technology can cause radioactive contamination in the ecosystems. In particular, cesium (Cs) has problems, such as neurological diseases, when it is exposed and accumulated in the bodies of animals, plants, and humans for a long time. Therefore, the development of simple and economical adsorbents for Cs removal is required. In this study, the surface of petroleum residue pitch was modified using NaClO and it was used to remove Cs from an aqueous solution. Batch experiments and characterization of the modified adsorbent were performed to determine the adsorption mechanism between the adsorbent and Cs. From these results, chemical and monolayer adsorption were found to occur at the carboxyl groups on the adsorbent surface, along with a cation exchange reaction occurred due to the sodium ions on the surface. Through this modification process, the total acidity, including phenolic, lactonic and carboxylic functional groups, was improved to 1.563 mmol/g and the maximum adsorption capacity of Cs for the modified adsorbent was 65.8 mg/g.