Chlorosaccus fluidus is a freshwater alga in the class Tribophyceae (also known as Xanthophyceae, or yellow-green algae), within the phylum Ochrophyta. The yellow-green algae are a diverse group of heterokont algae distinguished from green algae by their lack of chlorophyll b, the presence of chlorophyll c alongside chlorophyll a, and their typically yellow-green colouration resulting from the dominance of the accessory pigment diadinoxanthin. The genus Chlorosaccus encompasses simple, saccate (bag-like) colonial forms found in freshwater habitats such as ponds, ditches, and slow-moving streams, where they form gelatinous masses attached to submerged surfaces or floating freely. As a photosynthetic organism, C. fluidus contributes to primary production in its aquatic habitats and forms part of the periphyton community grazed upon by aquatic invertebrates. The taxonomy of yellow-green algae is complex and has undergone revision with molecular phylogenetic analysis. As a microalgae, C. fluidus has no common name and is listed as Not Evaluated by the IUCN. Its ecological importance lies in its role as a primary producer and in its contribution to the complex algal communities that characterise healthy freshwater ecosystems across the temperate Northern Hemisphere.