Chroomonas placoidea is a species of cryptophyte alga in the family Chroomonadaceae, documented from freshwater and aquatic environments in Europe. The specific epithet placoidea may refer to a flattened or plate-like aspect of the cell morphology, from the Greek plakos meaning flat plate. Cryptophyte cells are frequently compressed or dorsiventally flattened, and small differences in cell shape and proportions are important characters for distinguishing species within the morphologically subtle genus Chroomonas. Chroomonas species are characterized by small cell size, typically ranging from five to twenty micrometers in length, with two flagella of unequal length arising from a ventral depression. The plastid occupies much of the cell volume and contains photosynthetically active pigments including chlorophyll a, chlorophyll c, and phycobiliproteins. Chroomonas placoidea contributes to primary production in its freshwater habitat and is part of the diverse microalgal community of European lakes and ponds. Like other cryptophytes, it may engage in mixotrophy under some conditions. The species has not been assessed by the IUCN, consistent with the general treatment of microalgal organisms in conservation frameworks.