Plastic pollution in the marine environment has been identified as a global problem; different polymer types and fragment sizes have been detected across all marine regions, from sea ice to the equator and the surface to the deep sea. However, quantification of marine plastics debris in the size range of nanoplastics (<1 μm) and ultrafine microplastics (<10 μm) is not constrained, because such minuscule particles are challenging to measure. In this work, we applied a novel analytical assay using Thermal Desorption – Proton Transfer Reaction – Mass Spectrometry (TD-PTR-MS), which is suitable to detect and identify plastics in the nanogram range. From two stations in the Wadden Sea (the Netherlands), we measured nanoplastics directly from seawater aliquots, and from filters with different mesh sizes. Our results show the presence of Polystyrene (PS) and Polyethylene terephthalate (PET) nanopalstics as well as ultrafine microplastics in the Wadden Sea water column. The mass concentration of PS nanoplastics was 4.2 μg/L on average, indicating a substantial contribution of nanoplastics to the Wadden Sea's total plastic budget.