The floristic composition and spatial distribution of algal communities in 97 stations in the Tiber River and its tributaries were studied. A total of 128 taxa were found, belonging mainly to the Chlorophyta, Bacillariophyta and Cyanophyta. Filamentous algae (Cladophora glomerata, Vaucheria geminata, Oedogonium sp. and Spirogyra spp.) were the most common and abundant species. Melosira varians, Cocconeis pediculus, Gomphonema olivaceum, Cyclotella meneghiniana, Fragilaria crotonensis, Leptolyngbya valderiana and several species of Oscillatoria were also widely distributed but they were not important in terms of biomass. Spatial variations of species richness were found, with the middle and upstream sections showing a wider range of taxa than the downstream sections. Chamaesiphon incrustans and Lemanea fluviatilis were found only in the upstream sections of some watercourses of the basin in fast-flowing, cool, well-oxygenated waters, while Stigeoclonium tenue and Ulva flexuosa was recorded exclusively in downstream sections with slow-flowing, warm and eutrophic waters.