Mesostigma viride is a species of freshwater green algae. It is now considered to be one of the earliest diverging members of the Streptophyta, one of the two lineages of green plants (Viridiplantae).

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, or as close to Chaetosphaeridium. More recent studies agree that Mesostigma and Chlorokybus form a clade, being the earliest diverging streptophytes.