Mississippi Hydrochidae are poorly known. In this paper, we attempt to untangle some of the taxonomic difficulties caused by recent dubious works of D. Makhan and to validate an undescribed species from previously unpublished research. The 12 species occurring in Mississippi are reviewed and one new species, H. falsus Hellman sp.n., is described. Six new synonymies are established as follows: Hydrochus hanoewanti Makhan, 1994 is a junior synonym of Hydrochus rugosus Mulsant, 1844; Hydrochus monishi Makhan, 1995 is a junior synonym of Hydrochus foveatus Haldeman, 1852; Hydrochus equicarinatus Blatchley, 1928 is a junior synonym of Hydrochus simplex LeConte, 1855; Hydrochus daviniaae Makhan, 1995 is a junior synonym of Hydrochus vagus LeConte, 1851; Hydrochus bakkeri Makhan, 1995 is a junior synonym of Hydrochus obscurus Sharp, 1882; and Hydrochus soesae (Makhan), 2004 is a junior synonym of Hydrochus scabratus Mulsant, 1844. Hydrochus inaequalis LeConte, 1855 is removed from synonymy with Hydrochus foveatus Haldeman, 1852 and resurrected as a proper species. Neotypes are designated for H. rugosus Mulsant, 1844, H. rufipes Melsheimer, 1844, and H. scabratus Mulsant, 1844. A lectotype is designated for H. foveatus Haldeman, 1852. Species descriptions, illustrations of male genitalia, distribution maps and a key to species are presented. Additional notes on Makhan species and holotypes, including unavailable types, are provided.