Monoraphidium is a genus of green algae in the family Selenastraceae.

[2] Monoraphidium is found free-floating or attached to surfaces in water, or in soils.

[1] It is one of the most common types of phytoplankton in freshwater habitats, and has a cosmopolitan distribution.

The cell is straight to lunate to sigmoid or helically shaped.

[1] It is similar to and often confused with the related genus Ankistrodesmus, from which it differs by being typically unicellular.