In Korea, members of the genus Tettigonia have been known as two species, T. ussuriana and T. dolichoptera dolichoptera. However, the taxonomic status of the Jeju Island population of T. ussuriana (JJ-TU) is in question, relative to the mainland population (ML-TU), because of their different body sizes and ratios of wing length. To clarify the relatedness of JJ-TU and ML-TU, we examined the genetic variation and phylogenetic relationships within and between T. ussuriana and related species collected in South Korea, using five genetic loci: three mitochondrial genes (cytochrome c oxidase subunit 1 [CO1], cytochrome c oxidase subunit 2 [CO2], NADH dehydrogenase 1 [ND1]) and two nuclear loci (second internal transcribed spacer [ITS2], and tubulin alpha-1 [TA1]). Unexpectedly, the JJ-TU population is explicitly sister to T. d. dolichoptera, with low genetic distance (0.76-1.22% in CO1), indicating no direct connection with the ML-TU population; this finding suggests a recent divergence involving rapid morphological change without gene flow between JJ-TU and mainland T. d. dolichoptera. The separation of these populations from their common ancestor was caused by geographical isolation during last glacial age. This finding indicates that the JJ-TU population should be elevated to the rank of subspecies, at the very least. Furthermore, the ML-TU population was also revealed to have four genetically divided groups (group A-D) from four localized populations, but no significant morphological differences exist among them. The genetic difference (range 3.19-4.10% in CO1) between group A + B and C + D was especially large, suggesting that cryptic speciation has widely occurred within the mainland areas, caused by allopatric isolations resulting from mountain barriers.