Kernel size, one of the traits that determine wheat yield, is controlled by multiplequantitative trait loci. Polish wheat (Triticumpolonicum) has elongated and plump kernel and is a valuable material for breeding high-yielding wheat cultivars. However, genes or loci determining kernel length (KL) in Polish wheat are unknown. We identified and validated a major KL gene,KL-PW, at theP1locus in Polish wheat.KL-PWisVRT-A2, which encodes a MIKC-type MADS-box protein (MADS55). An insertion/deletion mutation in intron 1 ofVRT-A2^PWled to an alternatively spliced transcript,VRT-A2^PW2.Quantitative PCRanalysis showed thatVRT-A2^PWwas more highly expressed in developing seeds than wasVRT-A2^Ailanmai.Brassinosteroid(BR) sensitivity experiment and the expression of BR-related genes indicated thatVRT-A2^PWfunctions as a positive regulator of BR responses.VRT-A2^PWsignificantly increased KL of wheat. These findings not only reveal the molecular basis ofKL-PWin controlling KL, but also provide a valuable genetic resource for increasing kernel size in wheat.