<p>This study investigates the spatial and seasonal distribution of microplastics in Borçka Reservoir, located in the Eastern Black Sea Basin (Türkiye), based on field sampling conducted during winter 2023 and spring, summer, and autumn 2024. The samples (water–sediment) were collected seasonally from multiple stations and analyzed for microplastic abundance, morphology, size, color, and polymeric characteristics. Microplastic abundance in surface waters ranged from 0.14 to 0.31 items L⁻¹, with the highest concentrations observed in autumn 2024, while sediment concentrations varied between 8 and 32 items kg⁻¹ dry weight, exhibiting relatively limited seasonal variability. Fragments (38.1%) and fibers (35.1%) dominated the microplastic assemblage, indicating that secondary microplastics derived from the fragmentation of larger plastic debris represent the main contamination source. Most particles were within the 1–2&#xa0;mm size range (300–3310&#xa0;µm), and transparent (21.4%) and blue (20.8%) particles were the most common color categories. Polymer analysis revealed the predominance of PETG (20.8%), PP (11.3%), PA (10.1%), ABS (10.1%), and PE (9.5%), reflecting continuous land-based inputs related to packaging materials, textile fibers, fishing activities, and industrial sources within the catchment. Overall, the results indicate that Borçka Reservoir acts as an effective sink for microplastics and highlight the need for seasonally resolved monitoring and targeted management strategies to reduce land-based microplastic inputs in freshwater reservoirs, providing transferable insights for microplastic management in dam-regulated freshwater systems.</p>