Estuaries function as critical transitional zones governing microplastic (MP) transport from terrestrial catchments to coastal marine environments, yet bidirectional trapping mechanisms in tropical monsoon systems remain poorly characterized. This study quantified MP abundance, composition, and transport dynamics across catchment, estuarine, and offshore zones of the Nandu and Wanquan Rivers on Hainan Island, China, during three contrasting seasons: early wet season (April 2023), peak monsoon with typhoon disturbance (August 2023), and dry season (January 2024). The two systems represent contrasting estuary types, wave-dominated deltaic (Nandu) and bar-built lagoon (Wanquan), both of which trap MPs through salinity-driven flocculation and density stratification, though the semi-enclosed lagoon geometry of the Wanquan system extends particle residence time and amplifies retention efficiency. Catchment zones recorded the highest MP concentrations (closest to terrestrial sources), with estuaries functioning as effective filtration zones that reduce suspended MP loads by 86.6–97.6% before offshore release. A linear mixed model explained 96.0% of total variance (R² = 0.960), with zone as the dominant fixed effect. Generalized additive models identified salinity (peak accumulation at 1–15 ppt) and temperature as the primary non-linear retention drivers. PERMANOVA identified zone and season as dominant compositional structuring factors. A tidal mixing model revealed that marine-origin MP contributions were negligible under normal conditions but rose to 12.90–24.93 items m⁻³ during the August typhoon, with tidal import reaching 3.11–3.60 million items per cycle. Despite a ~190-fold offshore concentration surge, trapping efficiency remained high, indicating these tropical estuaries as robust salinity-controlled MP filters.