The thrips Scirtothrips dorsalis Hood (Thysanoptera: Thripidae) is an invasive pest that poses a significant economical threat to U.S. agriculture and trade. In this study, DNA sequence data and polymerase chain reaction (PCR) were utilized to develop a molecular diagnostic marker for S. dorsalis. The DNA sequence variation from the internal transcribed spacer 2 (ITS2) region of nuclear ribosomal DNA (rDNA) was analyzed from various thrips species, including S. dorsalis. A primer set and polymerase chain reaction cycling parameters were designed for the amplification of a single marker fragment of S. dorsalis ITS2 rDNA. Specificity tests performed on ten thrips species, efficacy tests performed on fifteen S. dorsalis populations, and tests on primer sensitivity and robustness all demonstrated the diagnostic utility of this marker. This diagnostic PCR assay provides a quick, simple, and reliable molecular technique to be used in the identification of S. dorsalis.