Chroomonas extensa is a species of cryptophyte alga in the family Chroomonadaceae, documented from freshwater environments in Europe. The specific epithet extensa refers to an extended or elongated cell morphology relative to other Chroomonas species. Cell shape and proportions are important diagnostic characters within the genus Chroomonas, where species are distinguished by cell dimensions, color, flagellar arrangement, and the structure of the ejectosomes — coiled proteinaceous organelles unique to cryptophytes that can be discharged rapidly, perhaps for predator avoidance. Cryptophytes of the genus Chroomonas are important primary producers in freshwater phytoplankton communities, particularly in cold-water periods when they may contribute substantially to total algal biomass. They are also important food sources in microbial food webs, supporting populations of heterotrophic flagellates and ciliates that consume them and transfer energy to higher trophic levels. Chroomonas extensa has been recorded from European freshwater localities. Like other microalgal species, it has not been assessed by the IUCN under formal conservation criteria.