Rivers play a major role in the transport of plastic debris from inland sources such as urban areas into the marine environment. The present study examined plastic particle concentrations and loads (>500 μm) upstream and downstream of an urban subcatchment over 15 months and investigated the relationship between river water discharge (Q) and plastic concentration (C). The plastic particle concentration increases by 0.8 g/1000 m3 or 79 n/1000m3 from the rural to the urban subcatchment. In the rural subcatchment, C does not increase with increasing Q (p = 0.57), whereas a positive relationship between C and Q exists downstream of the urban catchment (p = 0.00003). Combined sewer overflows likely contribute additional plastic loads during high flow conditions. Based on the C–Q relationship, we estimate the total plastic export in 2016 from the entire catchment to be 3.0 × 106 n/year or 2.6 × 104 n/(km2 year) and 15 n/(cap year). Because of the positive C–Q relationship, 90% of the plastic load is transported during 20% of the time. The analysis of time-resolved plastic concentration data in rivers provides a data-driven tool to better estimate plastic loads and to better understand the catchment controls of plastic in rivers.