Reactive oxygen species (ROS) are important in regulating normal cell physiological functions, i.e., cell death, proliferation and differentiation. Redox modulation could have significant implications providing an opportunity for the development of new strategies to improve clinical therapeutic outcomes in the treatment of diabetes, hypertension, atherosclerosis, carcinogenesis, aging and infections. Here, we report a versatile synthetic method to produce three polyethylene glycol (PEG)-based nanocomposite hydrogels containing different graphene derivatives homogeneously distributed: graphene oxide (GO), reduced graphene oxide (rGO) and graphene nanoplatelets (GNP) by free-radical redox polymerization. The graphene-PEG nanocomposite hydrogels (GCH) were very stable at different pH and solvents. Incorporation of small amount (1% wt) of graphene additives led to enhanced mechanical properties, up to 2.5-fold increase in elastic modulus and higher thermal stability (53-62 degrees C). The swelling behavior strongly depended on the functionalization of the graphene additive and their interaction with PEG. Interestingly, incorporation of graphene additives conferred antioxidant/pro-oxidant activity to the hydrogel, with their radical scavenging activity depending on the nature of the radical and the graphene derivative. PEG-rGO and PEG-GNP showed the highest scavenging activity for 2,2-Diphenyl-1-picrylhydrazyl radical (DPPH center dot) and hydroxyl radical, respectively. In addition, PEG-rGO demonstrated peroxidase activity in the presence of H2O2. The three GCH proved biocompatible, with no effect on cell viability and proliferation of human bone marrow derived mesenchymal stem cells (hMSC). The results pave the way for the design of bioactive functional nanocomposite hydrogels for ROS-mediated applications. [GRAPHICS] .