The larval morphology, including chaetotaxy of the head capsule and head appendages, of Armostus ohyamatensis Hoshina and Satô, 2006 is described in detail. This is the first description of larval morphology for the genus Armostus Sharp, 1890. Adults and larvae were associated using DNA barcoding of a molecular marker, 658 bp of mitochondrial cytochrome c oxidase I. Larvae of Armostus Sharp resemble other megasternine larvae, but they have a unique feature within the known hydrophilid larvae, i.e., incompletely cylindrically sclerotized maxillary palpomere 2.