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Vast quantities of debris are beaching at remote islands in the western Indian Ocean, despite minimal or zero local sources of pollution. We carry out marine dispersal simulations, informed by observations, incorporating currents, waves, winds, beaching, and sinking, for both terrestrial and marine sources of debris, to predict where this debris comes from. Our results show that most terrestrial debris beaching at these remote western Indian Ocean islands drifts from Indonesia, India, and Sri Lanka, although we also find evidence that a large fraction of bottles polluting these islands may have been discarded from fishing vessels and ships transiting the Indian Ocean. Abandoned, lost or otherwise discarded fishing gear (ALDFG) associated with purse-seine fisheries likely originates from within the western Indian Ocean, but ALDFG from longline fisheries may be sourced from further afield. Debris accumulation rates at Seychelles are likely seasonal, peaking during February-April. This pattern is driven by monsoonal winds and may be amplified during positive Indian Ocean Dipole and El-Nino events. We make suggestions for future beach clean-up operations, and also recommend that observational campaigns intending to identify sources of debris accumulating at remote islands should either target beaches that are not subject to regular clean-ups, or monitor accumulation over multiple years. Our results underline the vulnerability of small island states to marine plastic pollution, and are a crucial step towards improved management of the issue. The trajectories used in this study are available for download, and our analyses can be rerun under different parameter choices.