Oltmannsiellopsis is a genus of marine colonial flagellate green algae in the Oltmannsiellopsidaceae family of Chlorophyta.[1] It was named in reference to the similar genus Oltmannsiella. It has three species, O. viridis, which forms four-celled colonies, O. unicellularis, which is single celled, and O. geminata, which forms two-celled colonies. In Japanese it is called Japanese: ウミイカダモ ().[2]