Membrane technologies for water treatment and desalination are increasingly developed and utilized to address the global challenges of water security and supply. Membrane-based separations can produce water with desirable qualities from a wide range of water sources, such as groundwater, seawater, brackish water, and wastewater. However, the membranes, which are typically made from polymers, are still restricted by their inherent limitations, including a permeability/selectivity trade-off and a high fouling propensity. Functionalized graphene (i.e., graphene oxide and its functionalized derivates) is regarded as a very promising nanomaterial for overcoming these limitations owing to its superior hydrophilicity, mechanical stability, bacteriostasis, and biocidal properties. The addition of a small amount of this nanomaterial (normally less 1 wt% based on the weight of polymer) into polymeric membranes significantly improves their physicochemical properties and provides new functionalities. In this review, the recent advances in the preparation of functionalized graphene and its integration into polymer membranes for water treatment are summarized. The influences of functionalized graphene on the separation performance, antifouling ability, chlorine resistance, and mechanical properties of membranes are revealed. Current challenges and future perspectives in the development of high-performance functionalized graphene-polymer membranes are also discussed.