Reverse electrodialysis (RED) as a technique to harvest salinity gradient power has drawn increasing attention in the past decades. The key component of the RED stack, the ion exchange membrane (IEM), is limiting the development of salinity gradient power generation because of its suboptimal properties. These properties include electrical area resistance, permselectivity, and ion exchange capacity. IEMs specifically designed for RED applications with enhanced physical and electrochemical characteristics would achieve better energy output efficiency and enable wider application of the technique. Many researchers have found that composite membranes created by introducing inorganic nanomaterials into an organic polymer matrix showed enhanced electrical conductivity and permselectivity, as well as other improved membrane properties for RED. Moreover, specifically designed nanostructures, such as nanopores and nanochannels, significantly improved the ion transport efficiency and therefore herald an even brighter future for salinity gradient power generation.