Atomosia arkansensis , new species, is described from specimens collected in blackland prairie in southern Arkansas, and Atomosia tibialis is reported the first time from North America north of Mexico. A new key to Nearctic Atomosia species is presented. Atomosia melanopogon and A. mucida are noted to be sexually dimorphic. In addition to more standard characters, the open or closed condition of cell r₅ and the length of the pedicel and flagellum relative to the length of the scape are used to distinguish similar species. Lectotypes are designated for Atomosia mucida, Atomosia puella, and Atomosia sayii. Atomosia echemon is synonymized with A. puella (new synonymy), and A. mucidoides is synonymized with A. sayii (new synonymy).