Plastic pollution has caused increasing global concern. Currently, model estimates of the riverine plastic inputs to the global oceans based on the concept of Mismanaged Plastic Waste (MPW) varied substantially, and no field measurements of riverine inputs were available. We conducted sampling at the eight major river outlets of the Pearl River Delta, South China with rapid economic growth and urbanization to provide field measured data for fine-tuning modeling results. Floating microplastics (MPs) were collected with a Manta net (mesh size of 0.33 mm) five times during 2018. Microplastic particles (0.3–5.0 mm) widely occurred in all sampling sites. The number and mass concentrations of MPs were in the ranges of 0.005–0.7 particles m–3 and 0.004–1.28 mg m–3 and were positively correlated with water discharges. The annual riverine input of MPs from the Pearl River Delta was estimated at 39 billion particles or 66 tons, which converts to 2400–3800 tons of plastic debris based on calculations described in Text S2. These values were substantially below the MPW-based model estimates (91,000–170,000 tons). The large difference between measured and modeling results may have derived from the large uncertainty in the MPW values assigned to the world’s countries/regions.