This U.S. Geological Survey (USGS) Data Release provides phytoplankton data collected from Cheney Reservoir, Kansas, during June 2001 through September 2020. All data are reported as raw calculated values and are not rounded to USGS significant figures. This data release was produced in compliance with the open data requirements as a way to make scientific products associated with USGS research efforts and publications available to the public. The dataset includes all routine and quality assurance/quality control samples collected at the long-term sampling site on Cheney Reservoir located near the dam (USGS station identification number 07144790). Phytoplankton were identified to the lowest possible taxonomic level and abundance (density) and biovolume are reported.