<p>The marine environment receives microplastics (MPs) from multiple pathways, including rivers, wind, and surface runoff. However, subterranean estuaries—mixing zones of terrestrial groundwater and seawater beneath the coastal seabed—have received little attention as potential conduits of MP transport. Submarine groundwater discharge (SGD), a key hydrological process in coastal regions, may represent an overlooked pathway for plastics entering the ocean. In this study, we investigated MP contamination in the subterranean estuary (STE) of Malpe, southwestern India, and quantified the flux of MPs delivered to the Arabian Sea via SGD. The results revealed an average abundance of 7.56 ± 6.45 MPs L⁻¹ in subterranean estuarine waters, with seepage rate identified as the primary driver of microplastic flux. Fibres dominated the assemblage (100%), particularly in the 1000–5000&#xa0;μm size class, and polyethylene was the most common polymer type (96.82%). On average, SGD from the Malpe STE transported 18.93 ± 10.25 MPs m⁻² h⁻¹ to the coastal Arabian Sea. The dominance of polyethylene fibres strongly suggests fishing activities as the main source, consistent with traditional fishing practices along the Malpe coast. This study provides the first quantitative evidence of MP flux through SGD in an Indian subterranean estuary, highlighting SGD as a hidden pathway of marine plastic pollution. Incorporating such overlooked vectors is essential for improving coastal MP budgets and for developing effective management strategies to address plastic transport in coastal ecosystems.</p> Graphical abstract <p></p>