Mesenchymal stem cell (MSC) transplantation is promising for repairing heart tissues post myocardial infarction (MI). In particular, paracrine effects of the transplanted MSCs have been highlighted to play major roles in heart regeneration by secreting multiple growth factors and immune-modulatory cytokines. Nevertheless, its therapeutic efficacy still remains low, which is strongly associated with low viability and activity of the transplanted stem cells, because the transplanted MSCs are exposed to high shear stress during injection and harsh environments (e.g., high oxidative stress and host immune reactions) post injection. In this study, we aimed to develop novel injectable MSC-delivery microgel systems possessing high anti-oxidant activities. Specifically, we encapsulated MSCs in graphene oxide (GO)/alginate composite microgels by electrospraying. To further enhance the anti-oxidizing activities of the gels, we developed reduced MSC-embedded GO/alginate microgels (i.e., r(GO/alginate)), which have the potential to protect MSCs from the abovementioned harsh environments within MI tissues. Our in vitro studies demonstrated that the MSCs encapsulated in the r(GO/alginate) microgels showed increased viability under oxidative stress conditions with H2O2. Furthermore, cardiomyocytes (CMs), co-cultured with the encapsulated MSCs in transwells with H2O2 treatment, showed higher cell viability and cardiac maturation compared to monolayer cultured CMs, likely due to ROS scavenging by the gels and positive paracrine signals from the encapsulated MSCs. In vivo experiments with acute MI models demonstrated improved therapeutic efficacy of MSC delivery in r(GO/alginate) microgels, exhibiting significant decreases in the infarction area and the improvement of cardiac function. We believe that our novel MSC encapsulation system with GO, alginate, and mild reduction, which exhibits high cell protection capacity (e.g., anti-oxidant activity), will serve as an effective platform for the delivery of stem cells and other therapeutic cell types to treat various injuries and diseases, including MI. Copyright © 2019 Elsevier Ltd. All rights reserved.