Hydrochus is the only living genus of beetle in the family Hydrochidae, which belongs to the superfamily Hydrophiloidea, and was formerly treated as a subfamily of Hydrophilidae.

The diet is only known for the larvae of one species, H. japonicus, which feed on annelid worms belonging the family Naididae.

[6] 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, Ponohydrochus, is known from the Early Cretaceous (Hauterivian) Khasurty locality in Russia.

The following list includes some of the species in the genus:[8] Data sources: i = ITIS,[10] c = Catalogue of Life,[11] g = GBIF,[12] b = Bugguide.net[13]