An allele of Pm2 for wheat powdery mildew resistance was identified in a putative Agropyron cristatum -derived line and used in wheat breeding programs. Powdery mildew (caused by Blumeria graminis f. sp. tritici, Bgt) is one of the most devastating wheat diseases worldwide. It is important to exploit varied sources of resistance from common wheat and its relatives in resistance breeding. KM2939, a Chinese breeding line, exhibits high resistance to powdery mildew at both the seedling and adult stages. It carries a single dominant powdery mildew resistance (Pm) allele of Pm2, designated Pm2b, the previous allelic designation Pm2 will be re-designated as Pm2a. Pm2b was mapped to chromosome arm 5DS and flanked by sequence characterized amplified region (SCAR) markers SCAR112 and SCAR203 with genetic distances of 0.5 and 1.3 cM, respectively. Sequence tagged site (STS) marker Mag6176 and simple sequence repeat (SSR) marker Cfd81 co-segregated with SCAR203. Pm2b differs in specificity from donors of Pm2a, Pm46 and PmLX66 on chromosome arm 5DS. Allelism tests indicated that Pm2b, Pm2a and PmLX66 are allelic. Therefore, Pm2b appears to be a new allele at the Pm2 locus. The closely linked markers were used to accelerate transfer of Pm2b to wheat cultivars in current production.