Salinity gradient energy can be directly converted to electrical power using reverse electrodialysis (RED) and other technologies, but the power densities have been too low for practical applications. Here, an integrated RED stack and flow battery (RED-FB) system was developed to efficiently capture and store salinity gradient energy using two different redox couples (ferrocyanide and 2,6-dihydroxyanthraquinone) in the electrolytes, at an energy density of up to 2.4 kWh/m³-anolyte. These solutions were then discharged in the flow battery at a power density of up to 3.0 kW/m²-anode, which was more than 1000 times that obtained in the RED stack using NaCl alone (2.8 W/m²-anode). Salinity gradient energy captured from the RED stack was recovered from the electrolytes as electricity with a 30% efficiency. The combined RED-FB system overcomes many limitations of previous approaches to capture and store salinity gradient energy from natural or engineered sources.