The Donacia subtilis species-group, comprising D. subtilis Kunze, D. fulgens LeConte, D. confluenta Say, and D. tuberculifrons Schaeffer, is newly defined. A key to species, and genitalic figures of males for the species are given. Types are designated for all names where necessary. Donacia cuprea Kirby, 1837 (lectotype female, BMNH, designated herein), the oldest name available for D. quadricollis, auctorum, is revalidated. Donacia quadricollis Say, 1827 (neotype male, MCZ, designated herein) is newly placed as junior subjective synonym of D. subtilis Kunze, 1818. Donacia confluenta Say, 1827 (neotype male, MCZ, designated herein) is removed from synonymy of D. subtilis Kunze on the basis of differences in male genitalic structure. Donacia magistrigata Mead, 1938, is transferred from junior subjective synonymy of D. fulgens LeConte, 1851 (lectotype male, MCZ, designated herein) to junior subjective synonymy of D. subtilis Kunze, on the basis of similarity of male genitalic structure. The collection of Ahrens is lost, therefore a neotype for D. aenea Ahrens, 1810, not Hoppe, 1795, is designated (CNC, male). Donacia aerea Lacordaire, 1845 (lectotype, male, BMNH, designated herein) remains as junior subjective synonym of D. subtilis Kunze.