An ultrasensitive, specific and label-free immunosensor was developed to determine the lowest alteration of neuron specific enolase antigen (NSE), the standard biomarker of lung cancer patients. This immunosensor was the first biosensor which was fabricated by utilizing epoxy-substituted-polypyrrole P(Pyr-Epx) polymer as an immobilization platform. The fabrication steps of the immunosensor were followed by using electrochemical studies (electrochemical impedance spectroscopy and cyclic voltammetry) and these techniques were applied to characterize the binding interactions. Scanning electron microscopy and atomic force microscopy analyses were performed to investigate the changes formed on the indium fin oxide electrode surface morphology and electrode surface structure. The optimization of the experimental parameters and the analytical performance of the proposed immunosensor were widely evaluated. Under optimum experimental conditions, the change in impedimetric signal was determined to follow the specific biointeraction between anti-NSE antibody and NSE antigen. The sensing ability of the proposed immunosensor for NSE detection showed a wide linear detection range of 0.02-7.5 pg/mL, with a low detection limit of 6.1 fg/mL. This impedimetric immunosensor also demonstrated highly repeatable and reproducible responses, and reliable results in the analysis of human serum samples with recoveries between 98.29 % and 102.81 %. In addition, the designed immunosensor could be used for detection of lung cancer biomarkers after simple dilution of serum samples with phosphate buffer.