Chroococcus cumulatus is a cyanobacterium in the family Chroococcaceae, found in freshwater and semi-aquatic habitats. Members of the genus Chroococcus are among the simplest organized cyanobacteria, consisting of spherical cells that divide by binary fission to form pairs or tetrads held together within layered gelatinous sheaths. Cyanobacteria are cosmopolitan prokaryotes that carry out oxygenic photosynthesis using pigments including chlorophyll a, phycocyanin, and phycoerythrin. Chroococcus species occur in diverse aquatic habitats ranging from standing freshwater bodies to damp soil surfaces, rock faces, and tree bark in humid environments. The genus has historically been characterized primarily on the basis of cell size, sheath structure, and colony organization, though molecular analysis has increasingly revealed cryptic diversity within morphologically defined groups. Chroococcus cumulatus has been documented from northern European freshwater habitats. As a microscopic prokaryote, it has not been assessed by the IUCN, and its ecological role centers on primary production at the base of aquatic food webs in its habitat.