In this study, to enhance the adsorption capacity of cesium ions, one of the radioactive wastes, an oxygen functional group was introduced onto the activated carbon electrode via an oxygen plasma treatment, and polyaniline (PANI) was electrodeposited to prepare a capacitive deionization (CDI) electrode. The PANI-electrodeposited activated carbon electrodes exhibited a significant improvement in CDI performance, with an increase of up to 1.4 times compared to the non-electrodeposited electrode. This improvement is attributed to the enhanced electrical conductivity and the promotion of interactions between the electrode and cesium ions due to the incorporation of quinonoid imine (=N-), benzenoid amine (-NH-), protonated amine (-N+-), and protonated imine (=N+-) on the electrode surface. Notably, the electrode that was electrodeposited with PANI for 6 minutes showed the highest salt adsorption capacity (49.68 mg/g) and salt adsorption rate (4.19 mg/min), even though the nitrogen functional groups introduced were less compared to the electrode deposited for 8 minutes. This result is due to the optimal balance between the effect of the nitrogen functional groups derived from PANI and the appropriate amount of PANI that does not block the electrode’s pores. Consequently, an activated carbon electrode with PANI electrodeposition is expected to serve as a promising CDI electrode for efficient cesium removal.