Plastic debris in the South China Sea (SCS) is an environmental problem shared by the countries that border it, yet quantifying its transport and tracing its origins remain difficult. Using a three-dimensional offline Lagrangian model (Ichthyop) forced by a historical ROMS reanalysis of SCS circulation, we run ten-year experiments in two complementary modes: forward experiments release density-evolving particles monthly from coastal runoff sources, with wind and time-varying density included, and follow their transport and fate; backward experiments use neutral particles released around Hainan Island to reconstruct the upstream origins of plastic arriving at the coast. The forward experiments reveal horizontal pathways that reverse with the monsoon—southwestward and alongshore in winter, northeastward in summer—with pronounced month-to-month and interannual variability and recurrent nearshore accumulation hotspots, while coastline geometry and bottom topography also recurrently influence basin-scale particle fate. As particle density evolves, transport is three-dimensional rather than strictly surface-confined. The backward experiments show that plastic arriving at coasts around Hainan is traced mainly to the northern and western nearshore margins of the basin; the dominant source sector shifts seasonally—westward and southwestward in summer, northeastward in winter and autumn—with winter and spring drawing substantially more on the central basin than summer; median upstream recovery time is about 9 days. Over the ten-year simulations, monsoon-driven reversals of horizontal particle pathways occur every year, but their magnitudes vary substantially from year to year—northeastward summer transport is stronger in El Niño years, whereas the 2020 La Niña year is overall weaker than El Niño years. The backward experiments are consistent: inferred sources in La Niña years more readily extend into the central basin, while the strong 2015–2016 El Niño period concentrates sources in the western nearshore belt; plastic arriving at the coast from summer releases traces to more westward sectors overall, and from winter releases to more northeastern sectors.Because these sources are distributed around the basin and span several bordering coastlines, SCS plastic pollution is inherently transboundary, calling for coordinated, physically informed source attribution and coastal management.