Microplastics, particles smaller than 5 mm, have been found in urban rivers, including the North Shore Channel in Chicago, where concentrations were as high as those in oceans and the Great Lakes. This study shows that wastewater treatment plant (WWTP) effluent is a major source of microplastics in rivers. High-throughput sequencing revealed that bacterial communities on microplastics were less diverse and had different taxonomic compositions compared to those in the water column and suspended organic matter. Several taxa, including plastic-degrading organisms and pathogens, were more abundant on microplastics. These findings suggest that microplastics in rivers serve as a distinct microbial habitat and may act as a novel vector for the transport of unique bacterial assemblages. Urban rivers are an overlooked but potentially significant part of the global microplastic life cycle. Microplastics can interact with organisms, leading to potential ecological impacts, including the transfer of pollutants and pathogens. The study highlights the importance of further research on microplastic abundance, movement, and ecological effects in rivers to inform mitigation strategies.Microplastics, particles smaller than 5 mm, have been found in urban rivers, including the North Shore Channel in Chicago, where concentrations were as high as those in oceans and the Great Lakes. This study shows that wastewater treatment plant (WWTP) effluent is a major source of microplastics in rivers. High-throughput sequencing revealed that bacterial communities on microplastics were less diverse and had different taxonomic compositions compared to those in the water column and suspended organic matter. Several taxa, including plastic-degrading organisms and pathogens, were more abundant on microplastics. These findings suggest that microplastics in rivers serve as a distinct microbial habitat and may act as a novel vector for the transport of unique bacterial assemblages. Urban rivers are an overlooked but potentially significant part of the global microplastic life cycle. Microplastics can interact with organisms, leading to potential ecological impacts, including the transfer of pollutants and pathogens. The study highlights the importance of further research on microplastic abundance, movement, and ecological effects in rivers to inform mitigation strategies.