Plastics are the most abundant form of marine debris, with global production rising and documented impacts in marine environments. Recent research highlights that diverse microbial species are found to colonize plastic surfaces within marine waters. However, the qualitative and quantitative composition of marine biofilms on plastics is widely unknown. Therefore, this study investigates dissimilarities of the microbial community composition between various synthetic polymers and glass (substrate). Samples of eleven different synthetic polymers most relevant in the marine environment are exposed to in situ conditions for 15 months in a seawater flow- through system at the Biologische Anstalt Helgoland (Helgoland, German Bight). Scanning electron microscopy revealed diverse community structures comprising prokaryotic and eukaryotic microorganisms. Community compositions were assessed using Automatic Intergenic Spacer Analysis (ARISA) and linked to the different polymer type applying multivariate statistical techniques. Results of the multivariate statistical analyses revealed significant differences of the community compositions between the different substratums. Furthermore, differences in the microbial community composition among types of synthetic polymers have been identified. This study documented the microbial colonization of synthetic polymers in the marine environment. Furthermore, it reveals that the composition of the microbial communities attached to plastics in marine waters varies with the plastic substrate type.