Large-sample datasets are essential in hydrological science to support modelling studies and global assessments. The present dataset compiles all freshwater quality data that is available under open data policy (CC BY 4.0 or equivalent) at the GEMStat database for global water quality (www.gemstat.org). It includes over 20,000,000 measurements on 608 water quality parameters, covering 13,660 stations in 37 countries over the time period from 1906 to 2023. GEMStat is operated by the GEMS/Water programme of the United Nations Environment Programme (UNEP) and hosted at the International Centre for Water Resources and Global Change (ICWRGC) and the German Federal Institute of Hydrology (BfG). The data in GEMStat is provided by National Hydrological Services of UN member states. The data available here at Zenodo consists of a zip archive with individual csv files for different parameter groups. Three csv files provide metadata information on stations (GEMStat_station_metadata.csv), parameters (GEMStat_parameter_metadata.csv) and analytical methods (GEMStat_method_metadata.csv). A text file (README_output_format.txt) gives information on the different entriy classes in the data files.