<p>Rivers are major pathways for terrigenous plastics to the ocean, yet the spatio-temporal variability of plastic flux remains poorly quantified. Here, we conducted annual field monitoring of plastic particle fluxes in the Yangtze River, accounting for both vertical and horizontal variations in plastic distribution within the estuary. Microplastics were ubiquitous but exhibited spatial and temporal heterogeneity. Microplastics abundance declined with increasing depth, and the central channel exhibited lower microplastic abundance compared to the left and right channels. Monthly trends were consistent across channels within the same water layer. Hydrological patterns primarily regulated microplastic occurrence, with microplastic abundance decreasing at higher flow rates, while particle size correlated positively with flow. We estimated an annual flux of 5.20×10¹⁵ microplastic particles, of which 74.6% were small particles, posing substantial ecological risks due to their high toxicity and ingestion potential.</p><p></p>