A newly naturalized plant, Carex scoparia Schkuhr ex Willd. var. scoparia, was found in Mt. Hanseok, Injegun, Gangwon-do, Korea. It belongs to sect. Ovales Kunth and had been known from North America, and was introduced into Europe and New Zealand. This species distinguished from related taxa in Asia, Carex maackii Maxim. and Carex leporina L., by pistillate scales lanceolate and having large perigynia. We propose the new Korean name, ‘Han-seok-sa-cho’, based on its first collecting place. We also provide descriptions for morphological characters, illustrations, photographs and keys to allied taxa.