Microplastics are plastic particles that are smaller than 5 millimeters and are often found as pollution in our waterways. These polymer particles are globally distributed and are a direct result of human activity. Because of their rigidity and durability, microplastics are an ideal substrate for enhanced microbial growth and biofilm development. While microplastics have been studied in various contexts, only few studies have characterized the microbial communities on different types of plastic particles, but no study has been done in the estuarine water. In this study, we exposed three different types of plastics (polypropylene, polystyrene, and polylactic acid) to the water of Baltimore’s Inner Harbor, along with a non-plastic glass control. We used both in situ and in vitro incubations to understand the development of biofilm communities on microplastics. Microbial communities were analyzed based on the 16S rRNA gene sequences. We found that microbial composition on biofilm is distinct from that in the surrounding water, and different microplastic types have a minor impact on the composition of biofilm communities. The similarity between microbial communities on plastic and non-plastic particles suggests that surface supports rather than material types could be more critical for biofilm formation. Succession of microbial communities on the microplastics and interesting bacterial groups were described. Isolation and microscopic observations were also applied in this study. The presence of phototrophic organisms like filamentous cyanobacteria and Auxenochlorella on microplastic biofilms is interesting, and little is known about their contribution to carbon fixation in the ocean. Biofilms formed on microplastic surfaces could potentially affect the ecosystems via different mechanisms, including local nutrient cycling and the transportation of invasive or harmful species. As plastic production and mismanagement continues to be pervasive in our society, it is paramount that we include biofilm development into the framework of general ecology in order to truly understand the impact of plastic pollution and safeguard our ecosystems.