Plastic pollution is pervasive in marine systems in the Anthropocene era, and this pollution is known to carry complex biofilm-forming microbial communities. However, it is unknown whether these biofilm communities are specific to plastic or if they merely reflect those naturally occurring in the environment. To address this question, floating microplastics and non-plastic particles were sampled from the surface water of three marine ecosystems: the Baltic, Sargasso and Mediterranean Seas; for each location we characterized both free-living and particle-associated water community composition using 16S rRNA sequencing. We found that plastic and non-plastic biofilm communities were similar to each other across this large geographical range but found no measurable effect of the plastic polymer type. We identified plastic-specific OTUs that were not found either on non-plastic particles nor in the surrounding waters. Twenty-six of the plastic-specific OTUs were globally ubiquitous; this provides novel evidence that plastic repeatedly enriches the same bacteria from the surrounding water in diverse locations instead of simply mirroring natural microbiota. That the majority of the ubiquitous plastic-specific OTUs were assigned to the Rhodobacteraceae family, known to contain numerous hydrocarbonoclastic bacteria, and were also highly similar to bacteria previously sequenced in relation to oil-spills, suggests that properties of plastic, such as the absorption and leaching of hydrocarbons, could be an important part of the enrichment of bacteria to plastic biofilms. These plastic-specific bacteria that are repeatedly enriched from the surrounding waters, and not detectable on non-plastic particles, therefore have the potential to shift the aquatic microbial communities in which they are found. If plastic pollution continues to increase, selection for plastic-specific bacteria across the global oceans has potential to affect food web and ecosystem dynamics.