Microplastics (¡ 5 mm in length) are currently a major environmental threat to marine and terrestrial habitats worldwide. Tracking the sources and fate of microplastics floating at sea has become challenging as they encompass a complex suite of different properties. The current study aims to explore the main anthropogenic drivers of floating microplastics in the Northwestern Mediterranean Sea by analysing a large spatio-temporal dataset at 6 stations located adjacent (4 km from shore) to the main river catchments and urban areas along the North-Catalan coast. A Manta Trawl (200 and 315 µm-mesh) towed by an inspection vessel was used for sample collection, which allowed an extensive characterization of ¿38,000 floating plastics and microplastics over a 6-year (2014-2016 and 2018-2020) time period. We assessed spatio-temporal differences regarding microplastic size, type, morphology, composition and color, and covered seasonal fluctuations in oceanographic settings, river dynamics and socio-economic factors, such as population density. We discover an exponential increase in plastic pollution in recent years and find maximum concentrations over 5,800,000 items km-2 near Barcelona city adjacent to the Besòs River. We found that whether polyethylene and polypropylene fragments and films are the main types of plastics found along the entire study area, strong evidence shows that polypropylene microbeads also highly contribute to the microplastic budget, suggesting that rivers and waste water treatment plants are one of the primary sources of microplastics, and are likely to be transported from North to South by the Northern Current. Our preliminary results show a significant inter-annual variation linked to the size distribution of microplastics and likely as a function of riverine influx and proximity of industrial and recreational areas. Our study aims at contributing to help us better understand the sources and spatio-temporal variability of microplastics in order to develop relevant policies for mitigation.