Powdery mildew is a serious disease caused by Blumeria graminis f. sp. tritici (Bgt) that critically threatens the yield and quality of wheat (Triticum aestivum L.).  Using effective powdery mildew resistance genes is the optimal method for controlling this disease.  Against the background of high genetic homogeneity among the modern commercial cultivars that are mainly derived from conventional interbreeding, the resistance genes from wheat relatives have especially prominent advantages.  Octoploid triticale, produced from common wheat and rye (Secale cereale L.) through distant hybridization, is a new synthetic species and valuable gene donor for wheat improvement.  In this study, we developed the wheat–rye line YT5 through the hybridization of octaploid triticale and two wheat lines.  YT5 was confirmed to be a 6RL ditelosomic addition and 1R (1B) substitution line using genomic in situ hybridization (GISH), multicolor fluorescence in situ hybridization (mc-FISH), multicolor GISH (mc-GISH) and molecular marker analysis.  Genetic analysis showed that the powdery mildew resistance in YT5 was derived from the rye chromosome arm 6RL.  After inoculation with different Bgt isolates at the seedling stage, YT5 had compound reaction patterns with both obvious spores and hypersensitivity, and it gradually became highly resistant until the adult-plant stage, thus showing a resistance response significantly different from the reported Pm genes from rye chromosome 6RL.  YT5 also showed promising agronomic performance, so it is expected to be an elite resistance donor for wheat improvement.  To promote the transfer of the chromosome arm 6RL of YT5 in marker-assisted selection (MAS) breeding, we selected and verified two 6RL-specific kompetitive allele-specific PCR (KASP) markers that can be applied to efficiently detect this chromosome arm in different wheat backgrounds.