In this work, a novel, ultrasensitive electrochemical immunosensor has been fabricated for the determination of gypican-3 (GPC3) in human serum. A disposable indium tin oxide (ITO) glass array was used as the working electrode on which pyrrole-α-carboxylic acid (Py-α-COOH) was electropolymerised. This approach provided a high content of surface-confined carboxyl groups suitable for the direct covalent binding of GPC3 antibody. In the immunosensor system, the “fluorescence microscopy” technique was first used for the characterisation of carboxyl groups. Meanwhile, the electrode was characterised by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). Using GPC3 as a model analyte, the immunoassay exhibited high sensitivity, acceptable stability and reproducibility with a wide linear range from 0.9 pg mL⁻¹ to 9 ng mL⁻¹ and an ultra-low detection limit of 0.3 pg mL⁻¹ (S/N = 3). The results for real sample analysis in human serum demonstrated that the newly constructed immunosensor array provided a rapid, simple and cost-efficient immunoassay with high throughput and sufficiently low detection limits for clinical applications.