Polymeric membranes have a great potential for water treatment applications. However, their inherent hydrophobicity results in membrane fouling, limiting their separation performance. Therefore, polymeric membranes are modified by a blending approach to improve their physicochemical properties, which in turn improve their separation characteristics. Graphene oxide (GO) nanosheet is a promising additive used for polymer blending. Owing to its chemical structure and functionalities, it is compatible with the host polymer, resulting in high-performing membranes for water treatment. It imparts several essential properties to the resulting membranes, such as enhanced hydrophilicity, negatively charged surface, improved water permeance, and antifouling properties. In this chapter, we will present the state of the art of the polymeric membranes modified by GO nanosheet and their derivatives (e.g., reduced GO, functional GO, and GO-based nanohybrid) as additives, how these additives improve the membrane properties, and the separation performance efficacy of the resulting polymeric mixed matrix membranes in water treatment applications.