Although recent studies indicate that fluvial systems can be accumulation areas for microplastics (MPs),the common perception still treats rivers and streams primarily as pure transport vectors for MPs.In this study we investigate the occurrence of MPs in a yet unnoticed but essential compartment offluvial ecosystems - the hyporheic zone (HZ). Larger MP particles (500–5,000 μm) were detected usingattenuated total reflectance (ATR) - Fourier-transform infrared (FTIR) spectroscopy. Our analysis of MPs(500–5,000 μm) in five freeze cores extracted for the Roter Main River sediments (Germany) showed thatMPs were detectable down to a depth of 0.6 m below the streambed in low abundances (≪1 particle perkg dry weight). Additionally, one core was analyzed as an example for smaller MPs (20–500 μm) withfocal plane array (FPA)- based μFTIR spectroscopy. Highest MP abundances ( 30,000 particles per kg dryweight) were measured for pore scale particles (20–50 μm). The detected high abundances indicate thatthe HZ can be a significant accumulation area for pore scale MPs (20–50 μm), a size fraction that yet isnot considered in literature. As the HZ is known as an important habitat for invertebrates representingthe base of riverine food webs, aquatic food webs can potentially be threatened by the presence of MPsin the HZ. Hyporheic exchange is discussed as a potential mechanism leading to a transfer of pore scaleMPs from surface flow into streambed sediments and as a potential vector for small MPs to enter thelocal aquifer. MPs in the HZ therefore may be a potential risk for drinking water supplies, particularlyduring drinking water production via river bank filtration.