Reverse electrodialysis (RED) is a technique that can capture electrical potential from mixing two water streams of different salt concentrations through permselective ion exchange membranes. Effective design of ion exchange membranes through structure optimization is critical to increase the feasibility of salinity gradient power production by RED. In this work, we present the preparation of organic–inorganic nanocomposite cation exchange membranes (CEMs) containing sulfonated polymer, poly (2,6-dimethyl-1,4-phenylene oxide), and sulfonated silica (SiO2–SO3H). The effect of silica filler size at various loading concentrations on membrane structures, electrochemical properties, and the RED power performance is investigated. The membranes containing larger fillers (70 nm) at 0.5 wt% SiO2–SO3H exhibited a relatively favorable electrochemical characteristic for power performance: an area resistance of 0.85 Ω cm2, which is around 9.3% lower than the resistance of the membranes with smaller particle fillers (15 nm). The power performance of this nanocomposite CEM in a RED stack showed the highest gross power density of 1.3 W m−2: 10% higher power output compared with the membranes containing small particle size and 21% higher than that of commercially available FKS membrane. The goal of the present work is to develop an effective design for tailor-made CEMs for RED applications. Thus, a further optimized combination of material properties and membrane structure appears to be a viable option for the development of nanocomposite ion exchange materials that could provide greater power production by RED.