Monostroma kuroshiensis Bast, a green alga in the division Chlorophyta, is a green seaweed endemic to Kuroshio Coast of Japan. This high-value seaweed is called Hitoegusa or Hirohano hitoegusa (ヒロハノヒトエグサ) in Japanese. Previously this algae was known in binomen Monostroma latissimum, but the latest scientific research based on multilocal phylogeny discovered that this is a new species. This algae is commercially cultivated in East Asia and South America for the edible product "hitoegusa-nori" or "hirohano-hitoegusa nori", popular sushi wraps. Monostroma oligosaccharides with degree of polymerization 6 prepared by agarase digestion from Monostroma nitidum polysaccharides have been shown to be an effective prophylactic agent during in vitro and in vivo tests against Japanese encephalitis viral infection. The sulfated oligosaccharides from Monostroma seem to be promising candidates for further development as antiviral agents. The Monostroma genus is the most widely cultivated genus among green seaweeds.