Microplastic (MP) debris is now a global concern. The Chao Phraya is the largest river in Thailand and transports MPs from terrestrial areas to the ocean. MP debris in its water and sediment were measured in March 2021, September 2021, and March 2022 in five provinces along the watercourse. Hydrological data were also collected to estimate the MP riverine flux between provinces. Size, shape, color, and types of MP polymers were observed, with sedimentation data collected for MP content. Results showed that MPs were found at all sample stations, with average abundance in all province water samples ranging from 0.54 ± 0.05 to 1.07 ± 0.28 pieces/L, while in sediment sample, numbers ranged from 183.84 ± 38.76 to 546.18 ± 86.84 pieces/kg dry weight throughout all seasons. Overall contamination and accumulation were similar between provinces but significantly different between seasons. Sizes of MPs in water varied between seasons with MPs in sediment mostly 330–5,000 μm (Kruskal–Wallis, P < 0.05). Sedimentation of MPs was different between seasons (Kruskal–Wallis, P < 0.05). The highest MP flux values discharged from Samut Prakan Province to the inner Gulf of Thailand were 1.83 × 105 and 1.60 × 105 million items/day in September 2021 and March 2022, respectively.

  • Highest microplastic (MP) riverine flux discharged into the sea was recorded as 183,000 million items/day.

  • Fiber MPs were abundant in both water and sediment samples, with fragment MPs significantly higher in sediment samples than in water samples.

  • Significant seasonal variations were found in water and sediment samples.

  • Water discharge and current velocity play important roles in MP contamination in riverine ecosystems.

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