In this work, a novel immunosensor has been developed using an electrochemically reduced graphene oxide (ERGO) modified glassy carbon electrode (GCE) for sensitive and label-free detection of glycated hemoglobin (HbA1c) used as the gold standard marker for glycemic control. The prepared immunosensor, using ferrocyanide/ferricyanide as the mediator, ensures an uncomplicated and economic method for the detection of HbA1c. The proposed immunosensor can directly record changes in the electrochemical signals when the immunoreaction occurs on the sensor surface. The specific antibody-antigen immunoreaction resulted in a reduction in the amperometric response of the mediator that diffused at the electrode surface. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) techniques were utilized to investigate the electrochemical characteristics of the immunosensor. Calibration curve was plotted via differential pulse voltammetry (DPV) technique and a linear relationship was obtained between the DPV response and HbA1c concentrations from 1 to 25%. Under the optimized condition, the GCE/ERGO/HbA1cAb immunosensor supplied high selectivity and sensitivity for HbA1c detection in real serum samples.