Plastic production has increased dramatically since its large-scale industrialization in the 1950s, leading to widespread environmental contamination and making marine plastic pollution one of the most pressing global environmental challenges. Approximately 80-90% of marine plastics originate from land-based anthropogenic activities, yet substantial uncertainties remain regarding their transport pathways, emission magnitudes, and environmental fate. This review synthesizes current understanding of land-to-sea plastic transport by integrating evidence on multi-media transport pathways, global emission estimates, and the distribution and fate of plastics in marine environments. Rivers are consistently identified as the dominant transport pathway, contributing more than 80% of plastic inputs to the ocean, whereas atmospheric deposition, despite accounting for only 1-10% of the total input, represents an important source of plastics to remote marine regions. Coastal tourism, fisheries, and shipping activities provide additional localized inputs. Existing approaches for estimating global plastic emissions to the ocean are primarily based on bottom-up and top-down frameworks, yet reported fluxes differ by up to an order of magnitude because of contrasting model assumptions, parameterization strategies, and the limited availability of field observations. Recent studies indicate that Human Development Index-based (HDI) models generally show better agreement with monitoring data than conventional Mismanaged Plastic Waste-based (MPW) approaches. Once entering marine systems, plastics are transported across multiple environmental compartments, from coastal waters to the open ocean, from surface waters to deep-sea sediments, and from tropical to polar regions. However, the observed abundance of floating plastics remains substantially lower than estimated input fluxes, giving rise to the long-standing “missing sink” problem. Emerging evidence suggests that biofilm development and microbially mediated calcium carbonate mineralization may promote particle sinking and long-term sequestration in deep marine environments. Despite considerable advances in monitoring and analytical technologies, major challenges remain in constraining global plastic emissions and environmental fate because of insufficient field observations and methodological inconsistencies. Future research should prioritize integrated multi-media monitoring, improved model validation, standardized methodologies, and coupled source-to-sink frameworks to better quantify land-to-sea plastic transport and support effective mitigation of marine plastic pollution.

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