The ion exchange membrane (IEM) is a primary component in the reverse electrodialysis (RED) system for salinity gradient power generation. A great challenge exists in the selection of appropriate membrane materials and the design of proper RED membranes to optimize such an energy-producing process. This work presents a novel and cost-effective method for preparing hybrid cation exchange membranes by incorporating two organic polymers with fine film-forming ability. The functionalized poly (2,6-dimethyl-1,4-phenylene oxide) (sPPO) polymer with sulfated polyvinyl alcohol (sPVA) is proved to have great potential as a candidate for applications in RED. The prepared membranes showed that an optimal range of sPVA (2–10 wt%) improved the permselectivity (up to 87%) and an area resistance (down to 1.31 Ω cm2), which is comparable to those obtained with commercially available FKS (Fumasep®, Germany) membranes. The hybrid membrane containing 5 wt% of sPVA achieved the highest gross power density at 0.46 W/m2. This study shows the potential of using organic-organic hybrid membranes for the RED power generation system.