Kernel size, one of the traits that determine wheat yield, is controlled by multiple quantitative trait loci. Polish wheat (Triticum polonicum) 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 the P1 locus in Polish wheat. KL-PW is VRT-A2, which encodes a MIKC-type MADS-box protein (MADS55). An insertion/deletion mutation in intron 1 of VRT-A2^PW led to an alternatively spliced transcript, VRT-A2^PW2. Quantitative PCR analysis showed that VRT-A2^PW was more highly expressed in developing seeds than was VRT-A2^Ailanmai. Brassinosteroid (BR) sensitivity experiment and the expression of BR-related genes indicated that VRT-A2^PW functions as a positive regulator of BR responses. VRT-A2^PW significantly increased KL of wheat. These findings not only reveal the molecular basis of KL-PW in controlling KL, but also provide a valuable genetic resource for increasing kernel size in wheat.