Invasive non-native species are one of the main threats to biodiversity. Recently, more than 30 non-native species were reported to have established in the four major rivers of South Korea, and introduction of non-native species is gradually accelerating. However, ecological information necessary for prevention and management of non-native species introductions and consequent spread is lacking in South Korea, and especially so for freshwater non-native species. In this study, we assessed eight non-native freshwater fishes established in the Geum River watershed [the Risk Assessment (RA) area] using the Aquatic Species Invasiveness Screening Kit (AS-ISK). Receiver operating curve analysis identified threshold values of 24.75 for the BRA (Basic Risk Assessment) and 35.75 for the BRA+CCA (BRA + Climate Change Assessment). Four species, namely Carassius cuvieri, Lepomis macrochirus, Micropterus salmoides and Oreochromis niloticus were ranked as ‘high risk’; whereas, the lowest scoring non-native species was Coreoperca kawamebari, which is not likely to be invasive in the RA area because it has not established self-reproducing populations and its distribution is currently narrowing across the country. The results of this study indicate that application of AS-ISK allows the assessment of potentially invasive non-native species and provides a framework of action for the development of management guidelines for non-native species and the conservation of native species (cf. ecological traits such as species competition, habitat preference, food resources, and reproduction). As a case study for the Geum River watershed in South Korea, it is recommended that additional screening studies should be carried out for the non-native species in the four major rivers of South Korea.