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As an important wild relative of wheat, Agropyron cristatum has been successfully used for wheat improvement. Currently, a few useful agronomic traits of A. cristatum, such as high grain number per spike and resistance to diseases, have been transferred into common wheat. However, the effective detection of small A. cristatum segmental introgressions in common wheat is still difficult. The objective of this study was to identify A. cristatum-specific single nucleotide polymorphisms (SNPs) for the detection of small alien segments in wheat. The transcriptome sequences of A. cristatum were aligned against wheat coding DNA sequences (CDS) for SNP calling. As a result, we discovered a total of 167,613 putative SNPs specific to the P genome of A. cristatum compared with the common wheat genomes. Among 230 selected SNPs with functional annotations related to inflorescence development and stress resistance, 68 were validated as P genome-specific SNPs in multiple wheat backgrounds using Kompetitive Allele Specific PCR (KASP) assays. Among them, 55 SNPs were assigned to six homoeologous groups of the P genome using wheat-A. cristatum addition lines, and 6P-specific SNP markers were further physically mapped on different segments of chromosome 6P in 6P translocation lines. The P genome-specific SNPs were also validated by Sanger sequencing and used to detect the P chromatin in wheat-A. cristatum cryptic introgression lines. Two SNP markers (Unigene20217-182 and Unigene20307-1420) were detected in two wheat-A. cristatum introgression lines that showed enhanced grain number per spike and high resistance to powdery mildew. Together, the developed P genome-specific SNP markers will accelerate the detection of large numbers of wheat-A. cristatum derivatives and will be helpful for marker-assisted transfer of desirable traits from A. cristatum into adapted wheat cultivars in wheat breeding programs. Keywords: KASP marker, Single nucleotide polymorphism, Agropyron cristatum, Triticum aestivum