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Bioplastics are promising alternatives to conventional plastics, but their potential entry into marine ecosystems highlights the need for a better understanding of their interactions with microbial communities, including their role in the plastisphere. Here, we characterized the early biofilm formation on traditional plastics and bioplastics using a mesocosm approach. We tested the hypothesis that distinct bacterial communities selectively colonize traditional and biodegradable plastics in the marine environment. Specifically, fragments of the petroleum-based plastic polypropylene (PP) and the bioplastics Poly(3-hydroxybutyrate)- hydroxyvalerate (PHBv) and polylactic acid (PLA) were submerged in Baltic Sea mesocosms for three weeks. Biofilm colonization, prokaryotic abundance, and community composition were assessed through scanning electronic microscopy analysis, epifluorescence microscopy and 16S rRNA gene metabarcoding, respectively. Biofilm development increased over time on both traditional and bioplastics, with photosynthetic organisms appearing after 3 weeks. However, prokaryotic abundance decreased over time except on PLA surfaces. Prokaryotic communities’ composition differed among biofilms formed on the different polymers. The microbial community associated with conventional plastic PP was more similar to that of the seawater in the control treatment, while biofilms on PLA and PHBv shared a higher degree of similarity with each other. These findings suggest that microbial communities selectively colonize different plastic types, with bioplastics supporting distinct and specific bacterial biofilm assemblages over three-week exposure. The great diversity observed in bioplastics, particularly PLA, suggests they may support more complex and potentially active plastisphere communities after only three weeks of exposure to the Baltic Sea.