This U.S. Geological Survey (USGS) Data Release includes phytoplankton data collected from four reservoirs in Kansas, during August 2020 through August 2022. All data are reported as raw calculated values and are not rounded to USGS significant figures. The dataset includes all routine and quality assurance/quality control samples collected at twenty-five locations within Marion Reservoir, Milford Lake, Perry Lake, and Hillsdale Lake. Phytoplankton were identified to the lowest possible taxonomic level and densities and biovolumes are reported. This data is part of a larger multi-agency project between the U.S. Environmental Protection Agency, National Aeronautics and Space Administration, National Oceanic and Atmospheric Administration, and USGS called the Cyanobacteria Assessment Network (CyAN). The goal of the CyAN project is to develop a satellite-based, early warning system to detect harmful algal blooms (HABs) in freshwater systems.