Acheilognathus rhombeus is a temperate freshwater fish belonging to the Acheilognathinae sub-family of the family Cyprinidae. It originates in Japan and the Korean Peninsula. It was originally described as Capoeta rhombea by Temminck & Schlegel in 1846. It is the type species for the Acheilognathus genus. When spawning, the females deposit their eggs inside bivalves, where they hatch and the young remain until they can swim.