Once a promising material for the future, plastic has exponentially become an environmental threat with ubiquitous distribution. The UN and the G7 have recognised marine litter and microplastic pollution to have negative effects on aquatic ecosystems and, potentially, on human health. Despite the scientific progress in this field so far, researchers are only starting to unveil the tip of the iceberg. After being released in the marine environment, plastics undergo weathering and biofouling processes shaped by environmental conditions that lead to the fragmentation of larger items into microplastics (MPs). These increase the difficulty of accurately estimating abundances and polymer types in aquatic ecosystems. MicroplastiX aims to overcome knowledge gaps concerning MP weathering, degradation and fragmentation processes. MicroplastiX distinctively develops a holistic approach combining field data with laboratory experiments; deploys a comprehensive quality assurance scheme; evaluates MP interaction with biota (including release of plastic chemicals and sorbed pollutants); creates an inventory of colonization taxa; assesses horizontal and vertical transport, gradients and temporal distribution, all which will contribute to advanced multiscale models to predict fate and pathways. An experienced research team from several countries and diverse backgrounds will study a wide geographical area of ecologic and economic relevance, including Brazil, France, Germany, Ireland, Italy, Spain and Sweden (including two regions of upwelling, which is a novelty). Efforts for a synergistic collaboration ensure successful achievement of scientific objectives. MicroplastiX will thus contribute to the UN Sustainable Development Goals towards healthy and sustainable marine ecosystems and its results will potentially serve as benchmark guidelines for future work on MPs dynamics in the Atlantic Ocean and Mediterranean Sea, while complying with European and international policies.