The morphology of rice grains represents one of the most vital agronomic characteristics, significantly impacting both grain productivity and the subsequent milling and nutritional quality of the crop. A comprehensive understanding of the genetic basis and molecular drivers of grain shape is vital for the targeted breeding of high-performance rice lines with consistent yield stability. To pinpoint the genomic regions influencing grain dimensions, we conducted a genome-wide association analysis across a panel of 231 distinct rice accessions, focusing on the discovery of loci associated with length and width. Our analysis revealed four consistent quantitative trait loci (QTLs) distributed across chromosomes 3, 4, and 11. Notably, grain length was associated with qGL3.1, qGL3.2, and qGL11. The first two were co-localized with GS3 and SMG3, respectively, whereas qGL11 likely constitutes a novel locus. One QTL, qGW4, which governs grain width, was found to co-localize with the gene OsOFP14. Haplotype analysis further revealed that the characteristic haplotypes of the candidate genes for qGL3.1, qGL3.2, and qGW4 were enriched in eight germplasm accessions (including Newbonnet, Skybonnet, and Lemont), all of which exhibit a slender-grain phenotype. This finding suggests that the specific combination of these characteristic haplotypes is a common genetic signature of slender-grain rice, serving as a potential gene combination for the targeted improvement of rice grain shape. Our results reveal valuable QTLs and candidate genes and highlight specific germplasm resources that can be readily applied in marker-assisted breeding to improve rice grain shape.