Plastics, especially microplastics (<5 mm in length), are anthropogenic polymer particles that have been detected in almost all environments. Microplastics are extremely persistent pollutants and act as long-lasting reactive surfaces for additives, organic matter, and toxic substances. Biofilms are microbial assemblages that act as a sink for particulate matter, including microplastics. They are ubiquitous in freshwater ecosystems and provide key services that promote biodiversity and help sustain ecosystem function. Here, we provide a conceptual framework to describe the transient storage of microplastics in fluvial biofilm and develop hypotheses to help explain how microplastics and biofilms interact in fluvial ecosystems. We identify lines of future research that need to be addressed to better manage microplastics and biofilms, including how the sorption and desorption of environmental contaminants in microplastics affect biofilms and how microbial exchange between microplastics and the biofilm matrix affects biofilm characteristics like antibiotic resistance, speciation, biodiversity, species composition, and function. We also address the uptake mechanisms of microplastics by consumers and their propagation through the food web.