Abstract: Introduction: In recent years, fibronectin (FN) has emerged as a promising early biomarker for breast cancer (BC) due to its increased expression in tumor microenvironments and its association with cancer progression and metastasis. Detecting FN levels in biological samples offers valuable potential for the early diagnosis and prognosis of BC, improving patient outcomes through timely intervention. We developed and characterized a portable electrochemical immunosensor for FN biomarker quantification in epithelial extracellular vesicles samples (EVs). Methods: In this work, a screen-printed carbon electrode (SPCE) was modified with a mesoporous nanomaterial (CMK-9/KIT-6). Specific monoclonal anti-FN antibodies were immobilized on the silica KIT-6 nanomaterial, so the cancer biomarker was detected using a competitive immunoassay method. Moreover, the carbon CMK-9 nanostructure increases the electroactive surface area, and therefore the sensitivity in the quantification. FN biomarker present in the EVs sample was allowed to compete with FN-horseradish peroxidase (HRP) conjugated for the specific recognizing sites of immobilized anti-FN monoclonal antibodies. After that, the enzyme in the presence of hydrogen peroxide catalyzes the catechol oxidation, whose back electrochemical reduction was detected on the nanostructured electrode at -0.1 V. In this system, the FN concentration in the EVs sample was indirectly proportional to the FN HRP conjugated, showing a higher current by amperometry. Results: The immunosensor demonstrated high selectivity and stability with a linear relationship observed in the 0-100 ng mL⁻¹ concentration range. Under optimized conditions, it achieved a detection limit of 6 pg mL⁻¹, significantly lower than that of a commercial ELISA kit (0.1 ng mL⁻¹). The accuracy and reproducibility were validated against ELISA using spiked EVs, yielding intra- and inter-assay coefficients of variation below 4.32% and 6.28%, respectively. Moreover, the correlation between the results obtained with our portable immunosensor and the reference ELISA shows a straight line with a slope of 0.999, indicating good agreement between the two methods. Conclusions: The proposed portable immunosensor provides a highly sensitive, low-cost, and reproducible alternative to traditional benchtop assays. While further clinical validation with patient cohorts is required, this platform represents a promising step toward decentralized point-of-care tools for breast cancer monitoring.