Increasing the potassium use efficiency (KUE) of crops is important for agricultural sus3 tainability. However, a greater understanding of this complex trait is required to develop new, high KUE cultivars. To this end, a genome-wide association study (GWAS) was applied to diverse rice (Oryza sativa L.) genotypes grown under potassium stressed and replete conditions. Using high stringency criteria, the genetic architecture of KUE was uncovered, together with the breadth of physiological responses to low-potassium stress. Specifically, 3 quantitative trait loci (QTLs) were identified, which contained over 90 candidate genes. Of these, the sodium transporter gene OsHKT2;1 emerged as a key factor that impacts on KUE based on (i) the correlation between shoot Na+ and KUE, and (ii) higher levels of HKT2;1 expression in high KUE lines.