Mythimna macrosaris is a moth of the family Noctuidae. It is endemic to Kauai, Oahu, Molokai and Hawaii. The larvae mainly feed on Baumea meyenii, but have also been recorded feeding on Paspalum conjugatum, bunchgrass and sugarcane. The caterpillar may often be found on the leaves of its host or hiding in the dead leaves and trash at its base. The caterpillars become full-grown about a month after hatching from the eggs. The full-grown caterpillar is 42–45 mm.