The non-dissociative and dissociative adsorptions of zwitterionic Gly on graphene oxide (GO) was studied in the framework of DFT using a cluster model approach. In this work, the interaction with an epoxy group of GO basal plane was mainly considered. As a comparison, the non-dissociative and dissociative adsorptions of neutral Gly were also taken into account. The non-dissociative adsorption modes for zwitterionic and neutral Gly conformers show binding energies of 12.2 and 14.4 kcal mol−1, respectively. These molecules are thought to remain over the GO surface due to attractive noncovalent interactions. Two dissociative adsorption modes, for Z-Gly and N-Gly, show smaller binding energies of 7.2 and 8.4 kcal mol−1, where the deprotonated species links strongly through a C–O or C–N covalent bond to the GO surface. The results obtained in the present theoretical approach to the glycine/graphene oxide system support the fact that glycine can be attached to epoxy groups of graphene oxide basal planes in addition to the anchoring on edge oxidation groups. In summary, we conclude that glycine can be used as a reducing agent as well as a functionalizer of GO sheets.