Marine plastic pollution is increasing. The Indian Ocean is understudied compared to the Pacific and Atlantic Oceans. This study investigates plastic pollution in the Southwest Indian Ocean using a multi-faceted approach that includes both floating (visual survey and manta trawls) and beach-collected plastics, assessing their concentration, composition, and origin. Through 19 oceanographic campaigns and 153 uninhabited beach surveys, a total of 101,055 pieces of marine litter were identified, with 95% being plastics. Floating macroplastics were predominantly found near remote island waters, particularly at Glorieuses (103 items.km-2). Meanwhile, an increasing gradient of floating microplastic concentrations was observed from 40°E (103 items.km-2) to 65°E (105 items.km-2) along 30°/33°S. High concentration of beached macroplastics where observed on the east coast of Madagascar and Tromelin. Mesoplastics were more abundant than macroplastics, on remote islands. Floating and beached plastic debris were mainly hard fragments, mostly made of polyethylene (floating, beached: 72%, 57%) or polypropylene (26%, 34%). The majority of macroplastics identified in the brand audit, was mainly mineral water food packaging (81%) from Southeast Asian manufacturers. Our results will inform national management and provide evidence to support international plastic treaty negotiations on legacy plastics.