Riverine transport from land based sources constitutes an important pathway of plastic particles to the marine environment. However, fate and transport models for nano-, micro- and millimetre (NMM) sized particles are lacking. Prognostic fate models can be used to assess retention in freshwaters, to assess exposure to aquatic organisms and to quantify transport to sea. Here, we present a novel model for NMM polymer particles implemented for the river Dommel. The spatially and temporally explicit model accounts for advective transport, homo- and hetero-aggregation, sedimentation-resuspension, polymer degradation and burial. Experimental data on particle behaviour as well as literature data are used to parameterize the model. Model behaviour was tested by varying particle radius from 30 nm to 5 mm, for two polystyrene emission scenarios; a point source scenario and a realistic scenario with a combination of diffuse and WWTP (waste water treatment plant) sources. It appears that particle size, biofilm formation and water turbulence have dramatic effects on the fate and retention of NMM sized polymer particles in the Dommel catchment and on the positioning of the accumulation hot spots along the river.