Chroococcus submarinus is a species of cyanobacteria in the family Chroococcaceae, documented from saline and brackish aquatic environments. The specific epithet submarinus indicates an association with marine or nearshore saline habitats, distinguishing this species from the many Chroococcus species found exclusively in freshwater. Cyanobacteria are notably tolerant of a wide range of salinities, and the genus Chroococcus includes species from freshwater, brackish, marine, and hypersaline environments. In marine and coastal habitats, cyanobacteria contribute significantly to nitrogen fixation, particularly in oligotrophic tropical and subtropical seas where they can fix atmospheric nitrogen into biologically available forms, subsidizing productivity. Chroococcus submarinus, like other members of the genus, consists of spherical cells occurring in pairs or small groups within gelatinous sheaths. Marine cyanobacteria are important components of coastal biofilms on rocky shores, seagrass meadows, and other benthic habitats. The species has been recorded from European coastal and semi-saline environments. As a prokaryotic microorganism, it has not been assessed under IUCN criteria, and detailed ecological studies specific to this species remain limited.