This record provides the raster dataset of season-stratified phytoplankton density for Erhai Lake for 2020–2021. The dataset was generated using an optically informed machine-learning framework and is intended to support the manuscript entitled “Season-Stratified Retrieval of Phytoplankton Density Using Optically Informed Machine Learning”. The shared files include the derived phytoplankton-density raster products and accompanying documentation necessary to interpret the dataset. These raster products are derived research outputs and do not include the underlying in situ water-quality observations obtained under third-party data-use restrictions. Users should cite this dataset DOI, https://doi.org/10.5281/zenodo.19346972, when using the raster products in future research.