The paper presents the results of research on the identification of microplastics (MPs) and the assessment of the ecotoxicological risk due to their occurrence in the studied environmental component. For this purpose, bottom sediment samples were collected from a small water reservoir located on the Crișul Repede River in Bihor District (Romania). In the studied material, as many of 11 types of commonly used plastics in industry and households were detected. Spatial variability in the abundant presence of MPs, diversity of polymers and particle size was observed. The highest concentration of MPs (including the most toxic ABS, PU, BR, PVC) was found in the reservoir inflow zone. In turn, the amount and toxicity of detected MPs in the sediments behind the reservoir dam was significantly reduced. In general, these observations indicate that MPs are not only effectively deposited in the bottom sediments of reservoirs, but also that only a small amount of them is transported further downstream. The results obtained constitute a valuable source of information on the migration of MPs in dam reservoir ecosystems and will allow for a better understanding of the processes responsible for their transport, accumulation, and thus the increase in the level of long-term pollution of MPs in these in-land ecosystems. This is key, among other things, for developing strategies to reduce the pollution of MP in dam reservoirs and to protect the organisms inhabiting them, and consequently human health.