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A three-dimensional hydrodynamic-antibiotic model is developed to investigate the transport and dilution of sulfamethoxazole (SMX) in the Upper Gulf of Thailand (UGoT). The simulation produced a spatially averaged annual mean SMX concentration of 0.58 μg m − 3 , which varied slightly between seasons assuming a temporally constant river SMX loading observed in August. In contrast, the horizontal distribution of SMX concentrations strongly varied with season because of the changing residual currents. In addition, SMX is diluted to concentrations lower than 10% of those in river waters a short distance offshore of the estuaries. To better understand this behavior, we examined the relationship between salinity and SMX concentrations in the UGoT. The annual budget demonstrates that 98% of SMX in the UGoT is removed by natural decomposition. As the concentrations of fluvial pollutants in the UGoT depend on their river loading and decomposition rates, functions were derived to predict pollutant concentrations and flushing times based on the river input flux and half-life. [Display omitted] • The concentrations of SMX in seawater and rivers around the UGoT were observed. • A 3D ocean model was developed to investigate the transport of SMX in the UGoT. • The dilution characteristics of SMX depend strongly on ocean currents. • The fluvial pollutant concentration is linearly related to salinity conditionally. • A prediction function for fluvial pollutant concentration in the UGoT is obtained. The distribution of fluvial pollutants in the sea is affected strongly by ocean currents and their concentrations can be predicted based on river loading and half-life. [ABSTRACT FROM AUTHOR]