Since many noctuid moth species are highly destructive crop pests, it is essential to establish proper management strategies, which primarily require accurate and rapid species identification. However, diagnosis of noctuid species in the field, particularly at the larval stage, is very difficult due to their morphological similarity and individual color variation. In particular, caterpillars of Spodoptera exigua (Hübner), Spodoptera litura (Fabricius), Spodoptera frugiperda (Smith), and Mamestra brassicae (L.) (Lepidoptera: Noctuidae) are hard to be identified by morphology and frequently found on the same host crops in the same season, thus requiring a reliable species diagnosis method. To efficiently diagnose these species, we identified species-specific internal transcribed spacer 1 (ITS1) sequences and developed two molecular species diagnosis protocols using ITS1 markers. The first protocol was multiplex conventional PCR in conjunction with subsequent gel electrophoresis for species identification based on amplicon size. The second protocol was based on multiplex real-time PCR using fluorescent dye-labeled primers for single-step diagnosis. Template genomic DNA (gDNA) prepared by the DNA release method was also suitable for both protocols as the template prepared by DNA extraction. The two protocols enabled rapid and robust species diagnosis using a single multiplex PCR step. Depending on laboratory instrumentation, one of the two protocols can be easily adapted for species diagnosis of the four noctuid caterpillars in the field, which is essential for establishing proper management strategies. The multiplex real-time PCR protocol, in particular, will facilitate accurate diagnosis of the four species in a single step regardless of template gDNA quality.