We present a novel concept for coupling energy storage and water desalination using an acid–base flow battery architecture. In this device, electrical energy is stored through the reversible generation of acid and base, while salt is simultaneously removed from a central salt chamber. The device operates with non-toxic, earth-abundant electrolytes - NaOH and HCl - and utilizes hydrogen as an efficient redox mediator, avoiding crossover of redox active species and enabling high reversibility. We demonstrate that the degree of desalination directly impacts the desalination flow battery's open-circuit voltage and internal resistance, with high efficiency achieved at partial desalination. At 7 mA cm⁻², the device desalinates 0.5 M NaCl by 31% with 90–97% ion removal efficiency and 50% water recovery. Modelling of specific energy consumption indicates values as low as 14–18 kJ mol(NaCl)⁻¹ are achievable using state-of-the-art membranes and compartment designs. This places the device performance in line with leading desalination flow batteries while unlocking additional value through energy storage using abundant chemicals. We propose its use in decentralized coastal grids powered by intermittent renewables, where it can balance energy supply for downstream processes while at the same time desalinating seawater. This work outlines a scalable sustainable approach to address the water-energy nexus using benign and abundant chemicals.