There is an urgent need to identify, understandand incorporate alleles that benefit yield in polyploidwheat. The rice OsGW2 gene functions as a negative regulatorof grain weight and width and is homologous to thewheat TaGW2 gene. Previously it was shown that transcriptlevels of the A-genome homoeologue, TaGW2-A1, arenegatively associated with grain width in hexaploid wheat.In this study we screened the tetraploid Kronos TILLINGpopulation to identify mutants in TaGW2-A1. We identifieda G to A transition in the splice acceptor site of exon 5which leads to mis-splicing in TaGW2-A1. We backcrossedthe mutant allele into tetraploid and hexaploid wheat and generated a series of backcross derived isogenic lineswhich were evaluated in glasshouse and field conditions.Across 13 experiments the GW2-A1 mutant allele significantlyincreased thousand grain weight (6.6 %), grain width(2.8 %) and grain length (2.1 %) in tetraploid and hexaploidwheat compared to the wild type allele. In hexaploidwheat, this led to an increase in spike yield since no differenceswere detected for spikelet or grain number betweenisogenic lines. The increase in grain width and length wasconsistent across grains of different sizes, suggesting thatthe effect of the mutation is stable across the ear and withinspikelets. Differences in carpel size and weight betweenalleles were identified as early as 5 days before anthesis,suggesting that TaGW2-A1 acts on maternal tissue beforeanthesis to restrict seed size. A single nucleotide polymorphismmarker was developed to aid the deployment of themutant allele into breeding programmes.