Chroococcus planctonicus is a species of cyanobacteria in the family Chroococcaceae, adapted to the planktonic lifestyle suggested by its specific epithet. Unlike some Chroococcus species that are primarily benthic or periphytic — attached to substrates — Chroococcus planctonicus is characteristic of open-water plankton communities in freshwater lakes and ponds. Planktonic cyanobacteria must maintain a position in the water column for optimal light exposure, and some have evolved gas vesicles to regulate buoyancy, though these are more developed in filamentous bloom-forming genera. Chroococcus planctonicus cells are spherical, occurring in pairs or tetrads within gelatinous sheaths, following the basic morphological plan of the genus. It has been documented from European freshwater systems including Swedish and Finnish lakes, where it contributes to the seasonal phytoplankton community. Cyanobacteria are important primary producers in freshwater ecosystems, particularly in summer when warm temperatures and stratified water columns favor their growth. However, Chroococcus planctonicus is not generally associated with harmful algal blooms, which are more characteristic of genera such as Microcystis and Anabaena. Its conservation status has not been assessed.