Two-dimensional (2D) materials with atom- or few-atoms-thick layers have emerged as building-blocks in the synthesis of the next-generation membrane-based separations. Generally, 2D material-based membranes display high permeation and high selectivity due to their unique structure composed of nanopores and nanochannels with extremely short transport pathways. In this review, the latest advances and ground-breaking research studies on 2D nanosheets for gas separation are highlighted with a focus on the different strategies in synthesizing 2D nanosheets, their assembly into thin membranes and the type of transport mechanism taking place in such membranes.