Mesostigma is a genus of unicellular biflagellate freshwater green algae, with a single species Mesostigma viride, covered by an outer layer of basket‐like scales instead of a cell wall.

[4][1] It is the only known genus in the class Mesostigmatophyceae[5] (and thus, the only genus in the family Mesostigmataceae[2] and the order Mesostigmatales[3]).

Earlier studies were unable to resolve the position of the species, and it was often placed as a sister to all other green algae, as one of the basal members of the Streptophyta, close to Chaetosphaeridium.

[6][7][8] More recent studies agree that Mesostigma and Chlorokybus form a clade, being the earliest diverging of the green plants, specifically within Charophytes.

This green algae-related article is a stub.