Desalination technologies have heavily been investigated to utilize the abundant salt water on Earth due to the global freshwater shortage. During recent years, the desalination battery (DB) has attracted attention for its low-cost, eco-friendly, and energy-efficient characteristics. However, the current DB system is subject to inevitable performance degradation because of the mass-transfer limitation at the electrode–electrolyte interface, particularly when the system is used to treat brackish water. Here, we present a novel strategy to overcome the intrinsic mass-transfer limitation of DB in brackish water using an effective cell design based on a multichannel flow system. Compared to the conventional DB that consists of one feed channel, the multichannel desalination battery (MC-DB) is configured using two side channels introducing a highly concentrated solution to the electrodes and one middle feed channel for water desalination. The MC-DB showed a desalination capacity of 52.9 mg g^–1 and a maximum salt removal rate of 0.0576 mg g^–1 s^–1 (production rate of 42.3 g m^–2 h^–1) when a salinity gradient between the feed streams in the middle (10 mM NaCl) and side (1000 mM NaCl) channels was present, which were 3-fold higher than those in the case with no salinity gradient. In addition, the high concentration solution in the side channel significantly enhanced the rate capability of MC-DB, allowing the system to operate under a high current density of 40 A m^–2 with a desalination capacity of 34.1 mg g^–1. Considering the effect of electrolyte concentration on the battery electrode performance through electrochemical characterization, the highly saline medium at the side channel in the MC-DB creates an optimal environment for the battery electrode to fully capitalize the high desalination capacity, salt removal rate, and capacity retention of the battery electrodes.