Although DNA taxonomy is readily available, morphological keys are still valuable for quick and easy identification of species on site. In Korea, Hyla japonica is widespread throughout the country, whereas Hyla suweonensis occurs in the lowlands of western central Korea. H. suweonensis is rapidly disappearing and was consequently designated as critically endangered by the Korean government. We measured 19 characters for male individuals of the two tree frog species to develop a morphological key for identification. Our morphometric analyses indicated that the two tree frog species differed significantly in means of all morphological characters. In general, H. suweonensis was smaller and more slender than H. japonica. Moreover, the distributions of five characters related to head width and the angle between eyes and ipsilateral nostrils did not overlap in the two species and may be used for species identification. Because the character differences between the two species are small, all five characters should be used together to reliably distinguish the two tree frog species. Besides being used as a morphological key, our result in size difference leads to several research questions about microhabitat niche selection and competition between the two Korean tree frog species.