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Rice leaf width (RLW) is a crucial determinant of photosynthetic area. Despite the discovery of several genes controlling RLW, the underlying genetic architecture remains unclear. In order to better understand RLW, this study conducted a genome-wide association study (GWAS) on 351 accessions from the rice diversity population II (RDP-II). The results revealed 12 loci associated with leaf width (LALW). In LALW4, we identified one gene, <i>Narrow Leaf 22</i> (<i>NAL22</i>), whose polymorphisms and expression levels were associated with RLW variation. Knocking out this gene in Zhonghua11, using CRISPR/Cas9 gene editing technology, resulted in a short and narrow leaf phenotype. However, seed width remained unchanged. Additionally, we discovered that the vein width and expression levels of genes associated with cell division were suppressed in <i>nal22</i> mutants. Gibberellin (GA) was also found to negatively regulate <i>NAL22</i> expression and impact RLW. In summary, we dissected the genetic architecture of RLW and identified a gene, <i>NAL22</i>, which provides new loci for further RLW studies and a target gene for leaf shape design in modern rice breeding.