In this study, we report the development of a multiplex polymerase chain reaction (PCR) method using species-specific primers for the simultaneous detection of Poaceae members, including adlay, barley, maize, rice, and wheat, based on sequence polymorphism in DNA-directed RNA polymerase beta-prime chain genes (rpoC2). Species-specific primer pairs were constructed with a common forward primer and reverse primers differing by sequence polymorphisms and gene location. Each primer pair was designed to PCR-amplify products of five chloroplast genes of 443, 346, 278, 221, and 96 bp for rice, barley, adlay, wheat, and maize, respectively. Multiplex PCR with a series of template DNA concentrations (0.01–10 ng/μL) was used to optimize amplification of fragments from pooled Poaceae. In addition, species-specific primers were used to detect components of seven commercial flour-mixed products. This combination of the sensitivity of multiplex PCR with the specificity of primers designed to detect unique species has broad applications in the processed food industry.