Complex networks of above-ground roots and trunks make mangrove forests trap plastic litter. We tested how macroplastics relate to tree biomass, root abundance, mangrove geomorphology and river proximity, surveying landward and seaward margins of seven forests in the Philippines, a global hotspot for marine plastic pollution. Macroplastics were abundant (mean±s.e.: 1.1±0.22 items m-2; range: 0.05±0.05 to 3.79±1.91), greatest at the landward zone (mean±s.e.: 1.60±0.41 m-2) and dominated by land-derived items (sachets, bags), of which 99% were produced in the Philippines. Plastic abundance increased with proximity to river mouths, with root abundance only weakly influencing plastic weight and area. Tree biomass was unimportant. The study confirms rivers are a major pathway for marine plastic pollution, with mangrove roots, rather than tree density, are the biological attribute that regulate litter retention. The results suggest land-based waste management that prevent plastics entering rivers will reduce marine plastic pollution in Southeast Asia