Leeches in the genus Hirudinaria Whitman, 1886, also known as buffalo leeches, are blood-sucking ectoparasites of vertebrates. Although they are widely distributed in Asia and had been highly abundant in the past, studies on diversity and taxonomy of this genus are still scarce. There is probably a large amount of cryptic diversity yet to be discovered, particularly from mainland Southeast Asia. In this study, we used morphology and DNA barcoding with a COI gene fragment to explored the diversity of Hirudinaria leeches in the southern region of Thailand, where a unique geographic feature could have led to the diversification of freshwater biota. Molecular phylogenetic analyses and species delimitation approaches (ABGD, bPTP, GMYC, and BOLD) revealed the presence of four putative species of Hirudinaria leeches from southern Thailand, including H. bpling, H. thailandica, and two morphologically cryptic lineages of H. manillensis. Compared to other leech genera, genetic distances of Hirudinaria leeches were relatively low (0.11-0.65% within species; 3.72-14.36% between species) and barcoding gaps were very narrow (1.54-2.88%). The species diversity, distribution pattern, and a phenomenon of low genetic divergence of Hirudinaria leeches in southern Thailand could be explained by an ancient seaway, paleo-drainage, and anthropogenic activities.