Cascade reservoirs on the Drina River (Bosnia and Herzegovina) are heavily modified water bodies that require reliable biological tools for assessing trophic status and ecological potential. Under the Water Framework Directive (WFD), assessments of surface water ecological status and potential rely on biological quality elements, since aquatic communities integrate and respond to prevailing environmental conditions and thus serve as reliable indicators of water quality. This study aims to (i) describe phytoplankton diversity, biomass, and functional-group composition along the Drina reservoir cascade, (ii) examine monthly changes across the studied reservoirs, (iii) determine trophic status and ecological potential, and (iv) provide a preliminary estimate of total phosphorus thresholds that may support future setting of ecological potential boundaries. Phytoplankton composition and functional groups were analysed in three longitudinally connected reservoirs of the Drina River during four monthly surveys in 2024. A total of 80 phytoplankton taxa were recorded, with diatoms dominating most of the study period. The highest biomasses were recorded for Fragilaria crotonensis, Dinobryon divergens, Acanthoceras zachariasii and Sphaerocystis sp., while the dominant functional groups were P, E, A, and F. Phytoplankton assemblage structure showed moderate spatial differentiation among the reservoirs. Mean chlorophyll a and Carlson’s Trophic State Index indicated eutrophic conditions in the Višegrad Reservoir and mesotrophic conditions in the Perućac and Zvornik reservoirs, while biomass showed a pronounced summer maximum, particularly in Perućac. Ecological potential was generally classified as good or better, except for a moderate classification in the Zvornik Reservoir in late summer. The good/moderate TP boundary was estimated at 39 µg L⁻¹, linking EQR-based ecological assessment with the onset of eutrophic conditions. Overall, this study represents the first application of the phytoplankton functional group approach in cascade reservoirs in Bosnia and Herzegovina and may provide a valuable basis for the development of a phytoplankton-based monitoring framework in lakes and reservoirs, which is currently lacking.