The article presents the results of study of qualitative and quantitative indicators of phytoplankton of the Prut river within the Republic of Moldova in 2020-2022. In phytoplankton compositions of the river were identified 97 which belongs to 6 taxonomic groups and varieties of planktonic algae. It was established spatial and temporal fluctuations of the number and biomass of phytoplankton in Prut river. The numbers of phytoplankton changed within the limits 0,4326,14 mln. cell/l, with biomass 0,71-20,63 g/m3 in Prut river. On the basis of phytoplankton quantitative parameters, the trophic status and saprobiological state of Prut river was determined and their multiannual evolution was estimated. According to the phytoplankton biomass values river refers to the "eutrophic" trophic category, and in most cases, they were within the "mesotrophic" area. The values of saprobic indexes, estimated on the basis of species-indicators from phytoplankton composition, which are in proportion of 54% typically beta-mezosaprobic, confirm the following: the water quality of Prut river in the period of 2020-2022 years was satisfactory for the development of phytoplankton and was attributed mainly to II-III (good-moderately polluted) quality classes.