Macroalgae and macrophytes are important as habitats and spawning sites for many species of fish and invertebrates living in the Baltic Sea. The goal of our investigation to check whether growing macrophytes also concentrate and retain plastics, particularly that of microplastic (MP, 0.2-5 mm here) size range. Three summer expeditions in the southeastern part of the Baltic Sea (supported by the Russian Science Foundation, grant No. 19-17-00041) were conducted (July 30, August 5 and 7, 2019) in sea coastal zone, where communities of attached macroalgae (Furcellaria lumbricalis, Coccotylus truncatus, Polysiphonia fucoides, Cladophora rupestris, etc.) are developed on underwater boulders. Sampling of macrophytes was performed: (i) directly from growing thickets on underwater slope; (ii) by the diver working from the boat, in shallow coastal waters (floating torn off filaments); (iii) from the beach. Along with sampling of growing algae (from area 25×25 cm2) on the depths from 3.0 to 8.6 m, (distance from the shore - from 60 to 850 m), a hand pump was used to sample 20-100 liters of seawater from both algae thicket and algae-free water in surroundings. MPs were found in all the collected samples. Analysis shows on average 1.7 (in the range 1.1-5.3) times higher MPs contamination in water samples taken from the algae thickets (0.7-9 items per liter) than in those taken from the plant-free areas nearby (0.3-5.9 items per liter). Number of MPs per unit area (total) is in the range of 48-3088 items per m2. Fibres are the prevalent type of MPs in water and seaweed. Plant thalli are entangled by fibres. The majority of microparticles are fibres, mainly colorless and blue, but also red, black, golden, and yellow. Filamentous seaweed (Polysiphonia fucoides, Cladophora rupestris, Cladophora glomerata, and etc.) collect more fibres than cartilaginous Furcellaria lumbricalis and Coccotylus truncatus.