Capacitive deionization (CDI) has newly emerged as a desalination technology because of its energy and cost effectiveness. In particular, CDI using a flow electrode (FCDI) significantly increased salt removal by continuous desalination even at salt concentrations of both brackish and seawater. Since CDI mainly uses the electrosorption of salt ions onto an electric double layer on electrodes, carbon materials and their derivatives have been widely used. Even with various approaches, including modification and synthesis of new carbon-based electrode materials, the salt-removal capacity of CDI and FCDI is still limited by the accessible surface area and charge-transfer properties of carbon electrodes. Hydroquinone (HQ) is a redox-active organic molecule with highly reversible properties in electrochemical reactions and is a low-cost and non-toxic material. In this study, we introduced quinone in the activated carbon (AC) electrodes for CDI and FCDI desalination systems. It exhibited a significant increase in salt-removal performance (for CDI, more than a 65% increase for 300 mM HQ, and for FCDI, more than a 19% increase for 500 mM HQ). Even though the specific surface area of an HQ-added AC electrode is reduced because of the penetration of small HQ molecules into pores in the AC surface, the pseudocapacitive contribution from HQ and benzoquinone (BQ) increased the salt removal performance. We analyzed such electrochemical properties by using cyclic voltammetry and electrochemical impedance spectroscopy. We expect that such an approach will open a new door for realizing excellent deionization of saline water, even of seawater, and will have a strong potential for large-scale desalination and energy-storage systems.