The smooth cordgrass Spartina alterniflora, native to the east coast of North America and the Gulf of Mexico, was first found from Japanese tidal flats in 2008. The species has been reported from Aichi and Kumamoto Prefectures where it settles on tidal flats lower than native reed grass Phragmites australis and other saltmarsh plants. In 2014, the Ministry of Environment in Japan designated all Spartina species as the invasive alien species. While S. alterniflora has been exterminated extensively by local governments, it still has invaded other tidal flats in Japan. We review the situation of S. alterniflora in Japan with the comparison of morphological characteristics of S. alterniflora and P. australis native to Japan for early detection on Japanese tidal flats.