Starch is the most important component in the endosperm, and its synthesis is regulated by multiple transcription factors (TFs) in cereals. However, whether the functions of these TFs are conserved among cereals remains unclear. In this study, we cloned a B3 family TF in wheat, named TaABI19, based on its orthologous sequence in maize (Zea maysL.). Alignment of the DNA and protein sequences showed that ABI19is conserved in maize and wheat (Triticum aestivumL.). We found that TaABI19is highly expressed in young spikes and developing grains, and encodes a nucleus-localized transcriptional activator in wheat. The taabi19-b1null mutants obtained by EMS exhibited a down-regulation of starch synthesis, shorter grain length and lower thousand-grain weight (TGW). Furthermore, we proved that TaABI19 could bind to the promoters of TaPBFhomologous genes and enhance their expression. Haplotype association showed that TaABI19-B1is significantly associated with TGW. We found that Hap2and Hap3were favored and had undergone positive selection in China’s wheat breeding programs. Less than 50% of the modern cultivars convey the favored haplotypes, indicating that TaABI19still can be considered as a target locus for marker-assisted selection breeding to increase TGW in China.