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Summary Over‐application of potassium (K) fertilizer in fields has a negative impact on the environment. Developing rice varieties with high KUE will reduce fertilizer for sustainable agriculture. However, the genetic basis of KUE in a more diverse and inclusive population remains largely unexplored. Here, we show that the transcription factor OsNAC25 enhances K+ uptake and confers high KUE under low K+ supply. Disruption of OsNAC25 by CRISPR/Cas9‐mediated mutagenesis led to a considerable loss of K+ uptake capacity in rice roots, coupled with reduced K+ accumulation in rice and severe plant growth defects under low‐ K+ conditions. However, the overexpression of OsNAC25 enhanced K+ accumulation by regulating proper K+ uptake capacity in rice roots. Further analysis displayed that OsNAC25 can bind to the promoter of OsSLAH3 to repress its transcription in response to low‐ K+ stress. Nucleotide diversity analyses suggested that OsNAC25 may be selected during japonica populations' adaptation of low K+ tolerance. Natural variation of OsNAC25 might cause differential expression in different haplotype varieties, thus conferring low K+ tolerance in the Hap 1 and Hap 4 ‐carrying varieties, and the japonica allele OsNAC25 could enhance low K+ tolerance in indica variety, conferring great potential to improve indica low K+ tolerance and grain development. Taken together, we have identified a new NAC regulator involved in rice low K+ tolerance and grain development, and provide a potential target gene for improving low K+ tolerance and grain development in rice. [ABSTRACT FROM AUTHOR]