Graphene oxide (GO) membranes have exhibited great potential as emerging platforms for separation applications. The subnanometer-scale two-dimensional channels inside GO membranes make them ideal candidates of ionic separation membranes for both water–ion and ion–ion selectivity. Based on the fundamental understanding of water and ion transport behavior, we discuss the strategies, including d-spacing modulation, surface functionalization, and heterostructure creation for rationally regulating the microstructure of GO membranes for enhanced ion sieving properties. Furthermore, we highlight the existing challenges and prospects of GO membranes for ionic separation toward practical applications.