Under potassium (K)-deficient conditions, rice (Oryza sativa L.) actively takes up and utilizes sodium (Na) as an alternative element to K. In this study, we cloned a gene responsible for cultivar differences in shoot Na accumulation using a map-based cloning method. The responsible gene OsHKT2;1 encodes an Na transporter associated with Na uptake in root tissues, and its expression level was positively correlated with Na uptake potential in 11 rice cultivars. We found that OsHKT2;1 overexpression promoted shoot Na accumulation under low K supply and proposed that OsHKT2;1 expression level is a key factor in the Na accumulation potential in rice cultivars. However, under sufficient K supply, OsHKT2;1-overexpressing rice plants accumulated Na in roots but not in shoots. This result suggests that Na transfer from root to shoot may be regulated by another Na transporter.