The southern hemisphere water scavenger beetle genus Cylorygmus Orchymont, 1933 (Coleoptera: Hydrophilidae: Cylominae) is revised. Three species are recognized, one in Chile and two in South Africa. The morphological differences indicate that the African species are not congeneric with the Chilean one. Relictorygmus gen. nov. is established for the African R. trevornoahi sp. nov. (type species) and R. repentinus (Hebauer, 2002), both known from few localities in the Western Cape province of the Republic of South Africa. The genus Cylorygmus with the only species C. lineatopunctatus Orchymont, 1933 is endemic to a small region in central Chile. Its larva is described in detail based on specimens collected in association with adults. Both genera and all species are diagnosed, described and illustrated, and an identification key for adults is provided. Our study demonstrates that the trans-Atlantic disjunct distribution of Cylorygmus was based on inaccurate taxonomic treatment and did not reflect the real evolutionary history of these beetles.