Recently, the number of algological studies in Turkish inland waters has increased remarkably. However, taxonomic and floristic studies on algae in the Euphrates basin are still scarce. This study contributes new information to the knowledge of the Turkish freshwater algal flora. Phytoplankton samples were collected from the Tatar Dam Reservoir in the Euphrates Basin between January 2016 and December 2016. Two taxa were recorded for first time and 14 rare taxa for the freshwater algae of Turkey were identified in this study. The new records belong to the phylum Bacillariophyta, whereas taxa considered as rare belong to the phyla Chlorophyta, Cyanobacteria, Rhodophyta, Charophyta, Euglenophyta, and Bacillariophyta. The morphology and taxonomy of these taxa are briefly described in the paper and original light microscopy illustrations are provided.