<p>Extracellular vesicles (EVs) are central mediators of intercellular communication and disease progression, with important implications for diagnostics and therapeutics. EV concentration often increases in disease, making it not only a measure of experimental reliability and reproducibility but also a potential biomarker. Conventional techniques such as nanoparticle tracking analysis (NTA) and tunable resistive pulse sensing (TRPS) are widely used but are time-consuming, require specialized equipment, and are limited by EV heterogeneity. Thus, a faster, more scalable, and cost-effective method for EV quantification is needed. In this study, we developed an Extracellular Vesicle-Induced Silver-nanoparticle Aggregation (EVISA) assay for rapid and accessible measurement of EV concentration based on plasmonic nanoparticle aggregation with visible colour change. We screened multiple nanoparticles for aggregation upon mixing with EVs, assessed by naked-eye colour change and quantified with a standard ELISA plate reader. Transmission electron microscopy (TEM) confirmed nanoparticle aggregation, and zeta potential analysis demonstrated that EV-induced aggregation arose from destabilization of colloidal stability. Silver nanoparticles exhibited EV-induced plasmon coupling, evidenced by localized surface plasmon resonance (LSPR) shifts, resulting in increased absorbance at 450&#xa0;nm as detected by the plate reader. Together, these findings establish EVISA as a rapid, cost-effective, and scalable EV quantification platform that leverages widely available biological laboratory instrumentation, enabling broader adoption across the EV research community.</p> Graphical abstract <p></p>