This paper presents an assessment of the water quality of the transboundary Wisznia River (Poland). The environmental state of this transboundary river is described in terms of monthly monitoring data for the main physicochemical and biological parameters. The eight indicators of water quality taken into account included: nutrients (nitrate, phosphorus), salinity, and biological oxygen demand. Long-term water quality data (for 1990-2012), collected from two sample sites was investigated. A substantial decrease in the amount of nutrient substances in the later years can be noticed. This is the result of changes in agriculture and the construction of sewage treatment plants.