We found Diplachne fusca subsp. uninervia (J. Presl) P.M. Peterson & N. Snow, an unrecorded alien plant, in Gimpo-si, Gyeonggi-do and Ganghwa-gun, Incheon Metropolitan City. Diplachne fusca subsp. uninervia is native to North America (the USA and Mexico), Central America (Belize, Costa Rica, Honduras, and Nicaragua), the Caribbean (the Bahamas, Cuba, Jamaica, and Puerto Rico), and South America (Venezuela, Brazil, Bolivia, Ecuador, Peru, Argentina, Chile, Paraguay, and Uruguay). It is reported as an invasive species worldwide. Diplachne fusca subsp. uninervia is closely related to D. fusca subsp. fascicularis (Lam.) P.M. Peterson & N. Snow (Gaet-deu-reong-sae in Korean), an invasive species in Korea, and both are infraspecific taxa of the species D. fusca. Diplachne fusca subsp. uninervia is distinguished from D. fusca subsp. fascicularis by the small size of its spikelets, glumes, and lemmas, with apex awnless. Its common name is "Jom-gaet-deu-reong-sae" based on the short spikelets. We found that D. fusca (≡Leptochloa fusca), previously known as "Gaet-deu-reong-sae", was misapplied. It is morphologically different from D. fusca subsp. fascicularis. Therefore, we changed the scientific name of Gaet-deu-reong-sae from D. fusca (≡L. fusca) to D. fusca subsp. fascicularis based on the type specimens, original descriptions, and recent studies.