A polycytosine DNA-based immunosensor for electrochemical detection was developed and tested for detection of human epidermal growth factor receptor 2 (HER2), a breast cancer biomarker. We utilized gold nanoparticles (AuNPs) as supporting matrix to immobilize polycytosine DNA sequence (dC20) for electrochemical current generation and anti-HER2 antibodies. In the presence of target HER2, a sandwiched immunocomplex forms between a peptide specific to HER2 immobilized on the gold electrode and the anti-HER2 antibodies on the AuNPs. The HER2 captured by the sensor is detected because of the reaction of the dC20 phosphate backbone with molybdate, forming redox molybdophosphate precipitate that generates an electrochemical current on the surface of the electrode. The assay is sensitive: the calculated limit of detection of HER2 was as low as 0.5 pg/mL and the detection was linear to HER2 from 1 pg/mL to 1 ng/mL. The sensor's specificity is high, and there is no cross reactivity with several potential interferents, such as human IgG, human IgA, p53, carcinoembryonic antigen, and protein kinase. The sensor's performance with HER2 in clinical serum samples is similar to the performance of commercial ELISA assays. The configuration of polycytosine DNA as electrochemical current generating label and anti-HER2 antibodies on AuNPs is versatile and can be reconfigured to detect low levels of different analytes, or made more sensitive by amplifying the DNA to produce more phosphate to react with Na2MoO4.