We report on a combined experimental and theoretical study concerning the electrochemical behavior of the dexamethasone (DEX) on a graphene modified glassy carbon electrode (GCE). A good agreement between experiments and density functional theory (DFT)-based calculations is observed for the DEX reduction. The electrochemical behavior of the DEX was investigated on the surface of a glassy carbon electrode (GCE) modified with different type of graphenes, including graphene quantum dot (GQD), graphene oxide (GO), electrochemically synthesized graphene (EG), graphene synthesized by the Hummer method (HG) and graphene nanoplate (GNP) using voltammetric techniques (CV, DPV and SWV). The results exhibited a significant increase in the reduction of the peak current of the DEX in the GNP modified GCE (GNP/GCE) in comparison to other modified electrodes and bare GCE. The unique morphology, size and electro catalytic properties of the GNP cause a sensitive response of the DEX in a novel sensor. Under the optimized experimental condition, the GNP/ GCE showed two linear dynamic ranges of 0.1-50 μM and 50-5000 μM with a low detection limit of 15 nM for determination of the DEX. The novel sensor is successfully applied to the sensitive determination of the DEX in human plasma samples with satisfactory recoveries. Energy of the LUMO and HUMO orbitals and energy calculations for the DEX molecule interacting with graphene were performed using the density functional B3LYP/6-31G. The theoretical results allied to significant charge transfer took place due to the interaction of the DEX with the applied graphene.