<p>Prostate cancer is considered the third most diagnosed malignancy worldwide, and early diagnosis is crucial for effective treatment and improved patient outcomes. In clinical laboratories, specific antigen detection (PSA) testing within gray zone range of 2–4 ng mL⁻¹ remains challenging and typically relies on automated immunoassay analyzer, which are costly and require access to specialized laboratory facilities. Although qualitative lateral flow assays offer a low-cost alternative for use outside the laboratory, they lack the quantitative capability needed for early-stage cancer detection. In this study, we developed a portable and disposable electrochemical sensor for quantitative screening of total PSA. A carboxyl-functionalized graphene oxide (GO-COOH) layer was electrodeposited onto a flexible indium tin oxide–polyethylene terephthalate (ITO-PET) substrate to enhance conductivity and surface area, enabling sensitive biomarker detection. The biosensor exhibited a wide linear response range of 0–100 ng mL⁻¹ and a low detection limit (LOD) of 1.3 ng mL⁻¹. It demonstrated high selectivity and recovery value from 85.4% − 128.7%, with a relative standard deviation (RSD) below 15.5% in spiked serum samples. Overall, the developed GO-COOH@ITO-PET immunosensor successfully spans the clinically relevant cut-off levels, and offers strong potential as a low-cost, rapid, and quantitative platform for PSA screening in point-of-care settings.</p>