Microplastics, transported by rivers to oceans, are triggering environmental concern. This study aims to better understand river export of microplastics from land to sea. We developed the Global Riverine Export of Microplastics into Seas (GREMiS) model, a global, spatially explicit model for analysing the annual microplastics export to coastal seas. Our results indicate that riverine microplastics export varies among world regions, with several hotspots, e.g., South East Asia, and, depending on the 2050 scenario, may be doubled (Business as usual') or halved due to improved waste management (Environment profits'). Globally, our model simulations indicated fragmentation of macroplastics as the main source of microplastics, but this result heavily depends on the assumed fragmentation rate. Sewerage discharges contributed only 20 ranging from 1% (Africa) to 60% (OECD countries) and decreasing by 2050 as a result of improved sanitation. We conclude that, combating microplastics in the aquatic environment requires more region-specific analyses.