The OsGW2 gene is involved in rice grain development, influencing grain width and weight. Its ortholog in wheat, TaGW2, was considered as a candidate gene related to grain development. We found that TaGW2 is constitutively expressed, with three orthologs expressing simultaneously. The coding sequence (CDS) of TaGW2 is 1,275 bp encoding a protein with 424 amino acids, and has a functional domain shared with OsGW2. No divergence was detected within the CDS sequences in the same locus in ten varieties. Genome-specific primers were designed based on the sequence divergence of the promoter regions in the three orthologous genes, and TaGW2 was located in homologous group 6 chromosomes through CS nulli-tetrasomic (NT). Two SNPs were detected in the promoter region of TaGW2-6A, forming two haplotypes: Hap-6A-A (-593A and -739G) and Hap-6A-G (-593G and -769A). A cleaved amplified polymorphic sequence (CAPS) marker was developed based on the -593 A-G polymorphism to distinguish the two haplotypes in TaGW2-6A. This gene was fine mapped 0.6 cM from marker cfd80.2 near the centromere in a recombinant inbred line (RIL) population. Two hundred sixty-five Chinese wheat varieties were genotyped and association analysis revealed that Hap-6A-A was significantly associated with wider grains and higher one-thousand grain weight (TGW) in two crop seasons. qRT-PCR revealed a negative relationship between TaGW2 expression level and grain width. The Hap-6A-A frequencies in Chinese varieties released at different periods showed that it had been strongly positively selected in breeding. In landraces, Hap-6A-A is mainly distributed in southern Chinese wheat regions. Association analysis also indicated that Hap-6A-A not only increased TGW by more than 3 g, but also had earlier heading and maturity. In contrast to Chinese varieties, Hap-6A-G was the predominant haplotype in European varieties; Hap-6A-A was mainly present in varieties released in the former Yugoslavia, Italy, Bulgaria, Hungary and Portugal.