Millions of tons of riverine plastic waste enter the ocean via estuaries annually. The plastics accumulate, fragment, mix and interact with organisms in these dynamic systems, but such processes have received limited attention relative to open-ocean sites. In this Perspective, we discuss the occurrence and convergence of microplastics at estuarine fronts, focusing on their interactions with physical, geochemical and biological processes. Microplastic transformation can be enhanced within frontal systems owing to strong turbulence and interactions with sediment and biological particles, exacerbating the potential ecosystem impacts. The formation of microplastic hotspots at estuarine fronts could be a target for future plastic pollution mitigation efforts. Knowledge of the mechanics of plastic dispersal, accumulation and fate in frontal zones will, in turn, improve our understanding of plastic waste along the land–sea aquatic continuum.