Although rivers represent an important pathway for the transport of microplastics to the oceans, research on riverine microplastics is limited compared to the marine environment. Hence, we investigated the spatiotemporal distribution of microplastics in the Nakdong River down to 20 μm in size and characterized them using Fourier transform infrared spectroscopy in surface and mid waters and sediment. The mean (±standard deviation) abundance of microplastic in the Nakdong River was in the range of 293 ± 83 (upstream, February 2017) to 4760 ± 5242 (downstream, August 2017) particles/m3 in water, and 1970 ± 62 particles/kg in sediment. The abundance of microplastics was about three times higher in surface than mid waters in the downstream area. Polypropylene and polyester accounted for 41.8% and 23.1% of microplastics in the water, respectively, whereas about 50% in the sediment was composed of polypropylene and polyethylene. Microplastics smaller than 300 μm in size accounted for 74% in the water and 81% in sediment, and the distribution peaked in the 50–150 μm size range. Based on these results, we estimated the annual load of microplastics carried by the Nakdong River in 2017 to be 5.4–11 trillion by number and 53.3–118 tons by weight. The proportions of the total load transported through surface water and the water column were 8% and 92%, respectively. In addition, the microplastic load was concentrated in the wet season, which makes up 71% in number and 81% in weight. These results indicate that it is necessary to reflect seasonal variation and sample both in the surface water and water column to estimate microplastic transport. Without considering these factors, the annual load of microplastics may be overestimated or underestimated. Display omitted •The microplastic abundance was three times higher in surface than bottom water.•Five trillion microplastics were carried annually by the Nakdong River.•The microplastics were mainly transported by water column in the wet season.