Treated wastewater effluents are a source of emerging contaminants, including microplastics and pharmaceuticals, that impact rivers and streams. As microplastics are transported from wastewater treatment into the environment, pharmaceuticals can sorb to the surface and also be colonized by microorganisms. To investigate the microbial communities that are important in pharmaceutical transformation on microplastic surfaces, we used a culture-based approach with naproxen as the model pharmaceutical. Microplastic beads served as a solid substrate for delivering naproxen to anaerobic cultures inoculated with either anaerobic digester sludge or sediment from a wastewater-impacted river. After demonstrating naproxen transformation activity within the cultures, we separated the bulk liquid culture from the colonized microplastic beads and transferred them into separate bottles of sterile media amended with naproxen. Naproxen transformation occurred in cultures that contained microbially-colonized microplastics. Results from DNA analyses of the microbial community from each treatment demonstrated a different microbial community structure on the colonized plastic compared to that of the planktonic cells, thus illustrating a selection of the microbial community by the microplastics. These findings demonstrate that the microbial community attached to microplastic beads can continue pharmaceutical transformation activity when the microplastics are transferred to new media, thus serving as a model for the potential transport of pharmaceutical-transforming microbes from wastewater treatment to freshwater environments.