Pennisetum flaccidum Griseb. (Poaceae) is native to Central Asia. While monitoring Poaceae weeds nationwide in Korea, we found its new naturalized distribution in Gunsan, Jeonrabuk-do. The species is distinguished from P. alopecuroides var. alopecuroides by subsessile involucres and plumose bristles. We provide the first documented record of P. flaccidum Griseb. for Korea with the description and illustration. We also provide a taxonomic key to the species of Pennisetum in Korea.