Extreme environments, such as subterranean habitats, are suspected to be responsible for morphologically inseparable cryptic or sibling species and can bias biodiversity assessment. A DNA barcode is a short, standardized DNA sequence used for taxonomic purposes and has the potential to lessen the challenges presented by a biotic inventory. Here, we investigate the diversity of the genus Leptonetela Kratochvil, 1978 that is endemic to karst systems in Eurasia using DNA barcoding. We analyzed 624 specimens using one mitochondrial gene fragment (COI). The results show that DNA barcoding is an efficient and rapid species identification method in this genus. DNA barcoding gap and automatic barcode gap discovery (ABGD) analyses indicated the existence of 90 species, a result consistent with previous taxonomic hypotheses, and supported the existence of extreme male pedipalpal tibial spine and median apophysis polymorphism in Leptonetela species, with direct implications for the taxonomy of the group and its diversity. Based on the molecular and morphological evidence, we delimit and diagnose 90 Leptonetela species, including the type species Leptonetela kanellisi (Deeleman-Reinhold, 1971). Forty-six of them are previously undescribed. The female of Leptonetela zhai Wang & Li, 2011 is reported for the first time. Leptonetela tianxinensis (Tong & Li, 2008) comb. nov. is transferred from the genus Leptoneta Simon, 1872; the genus Guineta Lin & Li, 2010 syn. nov. is a junior synonym of Leptonetela; Leptonetela gigachela (Lin & Li, 2010) comb. nov. is transferred from Guineta. The genus Sinoneta Lin & Li, 2010 syn. nov. is a junior synonym of Leptonetela; Leptonetela notabilis (Lin & Li, 2010) comb. nov. and Leptonetela sexdigiti (Lin & Li, 2010) comb. nov. are transferred from Sinoneta; Leptonetela sanchahe Wang & Li nom. nov. is proposed as a replacement name for Sinoneta palmata (Chen et al., 2010) because Leptonetela palmata is preoccupied.