Hydrochus is the only living genus of beetle in the family Hydrochidae, which belongs to the superfamily Hydrophiloidea. Hydrochus includes about 180 species, which are found worldwide. Hydrochus has also been used for a fly genus in the family Dolichopodidae. This junior homonym is a junior subjective synonym of the genus Rhaphium. The oldest fossil of the modern genus is Hydrochus relictus from the Green River Formation of North America, dating to the Eocene. An extinct genus of the family, , is known from the Early Cretaceous (Hauterivian) Khasurty locality in Russia.