MXene nanosheets are 2D nanomaterials composed of transition metal carbides and nitrides. MXenes have recently emerged as innovative building blocks for next-generation high-performance gas separation membranes owing to their exceptional gas permeability and selectivity with outstanding physical and chemical properties. This paper provides a comprehensive review of the design and performance of MXene-based gas separation membranes. Furthermore, the characteristics and synthesis of MXene nanosheets are explained, and various types of MXene-based membranes, fabrication strategies, and gas separation mechanisms are introduced. Next, the gas separation performance of MXene-based membranes as reported in previous studies is evaluated. Critical challenges in designing commercial MXene-based gas separation membranes are also discussed. In addition, this review demonstrates the potential of MXene nanosheets for high-performance gas separation membranes.