Microplastics (MPs) are defined as synthetic organic pollutants with a size ¡ 5 mm, and have been recorded in various environments across the world. Due to their small size they pose a potential risk for many organisms throughout the food web. Yet, little is known about MP-distribution patterns and associated transport mechanisms into the oceans. River systems may act as pathways for MPs into marine environments, and especially the transition zone between these two environments is of great importance. In this study, we investigated the estuary and lower stretch of the second-largest German River Weser for the occurrence of MPs, being an important interface of fresh water and marine environments. Hereby, we are aiming at enhancing the general understanding by providing novel, comprehensive data and suggestions for future studies on riverine systems. Surface water samples of two different size classes (11-500 µm and 500-5000 µm) were collected by means of an on-board filtration system as well as net sampling. After a thorough sample preparation using state-of-the-art methods, MP samples were analyzed with µFTIR as well as FTIR-ATR in order to obtain information on concentrations, polymer composition and size distribution. Highest concentrations of MP were revealed in the sample fraction 11-500 µm, with the polymer cluster acrylates/PUR/varnish being dominant. Further, data showed that MP concentrations generally decreased from the river mouth towards the open sea. In the sample fraction 500-5000 µm, MP concentrations were lower, with polyethylene as dominant polymer type. This study contributes to the current MP research by providing novel insights into MP pollution of the estuary and lower stretch of an important German River, and provides implications for future MP monitoring and conservation measures.