In the process of pyrolyzing waste plastics, the generation of Clsub.2 gas can pose a problem. During the pyrolysis processing, incomplete combustion of organic compounds containing chlorine can lead to the formation of toxic chemicals, which can cause issues in subsequent processing stages. Therefore, an adsorbent plays an important role in removing Clsub.2 in the dechlorination process, and alkaline adsorbents and metal oxides are generally used. Waste red mud is composed of Fe metal oxide and alkaline components, so it is intended to be used as a Clsub.2 adsorbent. The Clsub.2 removal ability of red mud with different redox status of iron oxides was assessed. Hydrogen treatment was performed at various temperatures to control the reduction potential of the Fe in the metal oxides, and phase changes in the Fe oxide component of red mud were confirmed. In the case of red mud hydrogenated at 700 °C, most of the Fesub.2Osub.3 structure could be converted to the Fesub.3Osub.4 structure, and the Fesub.3Osub.4 structure showed superior results in Clsub.2 adsorption compared to the Fesub.2Osub.3 structure. As a result, red mud at an Hsub.2 treatment temperature of 700 °C showed about three times higher Clsub.2 adsorption compared to red mud without Hsub.2 treatment.