<p>The increasing demand to progress in the field of bio-sensor technology for the development of a specific bio-sensor in different fields of research for routine analysis has attracted a number of scientists for their interest in the field of biosensor research. The aim of this research to introduce the carbon paste electrode functionalized with ferrocene (FC) and multiwalled carbon nanotubes (MWCNTs) as label-free electrochemical impedimetric-immunosensor for the precise recognition of the most reliable prostate-cancer biomarker known as prostate-specific antigen (PSA). This immunosensor was constructed on the surface of the FC-MWCNTs/CPE electrode by immobilizing the primary antibody (Ab1) against the PSA. The fabricated FC-MWCNTs-CPE electrode was characterized by morphologically and electrochemically. The performance of the immunosensor was examined using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). EIS measurements were used to examine the immunoreaction between Ab1 and PSA at the immuno-electrode surface in the presence of [Fe(CN)₆]^3−/4− to detect PSA. Under the optimized conditions, the suggested immunosensor displayed a wide-linear concentration range (0.001ng/ml to 60 ng/ml) with the minimum detection limit of 0.8 pgml^− 1. The practical applicability of the immunosensor was investigated by checking its repeatability and reproducibility using human serum as real sample. A considerable repeatability and reproducibility was obtained for its determination in human serum, which confirms that the developed method is a good alternative for direct detection of PSA in biological samples with negligible interference. The carbon paste based immunosensor would open an avenue for the detection of various other biomarkers.</p>