Chroomonas diplococca is a species of cryptophyte alga in the family Chroomonadaceae, documented from freshwater and aquatic environments in Europe. The specific epithet diplococca suggests a morphological characteristic involving paired or double spherical cells or a cell shape reminiscent of diplococcus-like structures. Cryptophytes are distinctive eukaryotic algae characterized by the retention of a secondary endosymbiont-derived plastid bearing a nucleomorph — a miniaturized eukaryotic nucleus that persists in the periplastidial compartment. This unique evolutionary feature makes cryptophytes of considerable interest to researchers studying eukaryotic cell evolution and endosymbiosis. Chroomonas species contribute to primary production in freshwater food webs and are important prey items for heterotrophic nanoflagellates, ciliates, and copepods. In lakes and other freshwater bodies, cryptophytes often peak in abundance during cold-water periods such as spring and autumn when they can form a significant fraction of the phytoplankton biomass. Chroomonas diplococca has been recorded from European freshwater sites and is part of the diverse microalgal community of temperate freshwater ecosystems. It has not been assessed by the IUCN.