A new subgenus of the genus Schizoprymnus Foerster is described and figured from Honshu, Japan. The subgenus, Ibarakius subgen. nov., comprises three species, S. (I.) gotoi sp. nov. (type species), S. (I.) kaizawus sp. nov., and S. (I.) honshuensis sp. nov. A pair of long, curved posteroventral processes on the carapace is unique to Ibarakius subgen. nov. The variability of frontal protuberances and sutures of the carapace in the brachistine genera Triaspis Haliday and Schizoprymnus Foerster is discussed.