Cosmarium is a large genus of desmids (Desmidiaceae), a group of green algae closely related to the land plants (Embryophyta).

Cells are constricted in the middle, giving them a bilobed appearance;[5] the two halves are called semicells.

The cell wall may be smooth or ornamented with spines, granules, scrobiculations (pits), or pores.

Actinotaenium is generally circular in cross section, and Euastrum usually (but not always) has a notch at the tip of each semicell.

It is also not monophyletic, as molecular phylogenetic studies have shown that the form of Cosmarium evolved independently many times over.