The Japanese species of the genera Nesticella Lehtinen & Saaristo, 1980 and Howaia Lehtinen & Saaristo, 1980 (Araneae, Nesticidae, Nesticellini) are revised using an integrative taxonomic approach. Their morphology, phylogenetic position within the genera, assignment to species groups, and distribution in mainland Japan and the Ryukyu islands are discussed herein. A phylogenetic and species delimitation analysis was conducted to confirm the boundaries between the putative species that were initially discriminated based on external and genital morphology. As a result of the present study, six species of Nesticella and three species of Howaia are proposed for the fauna of Japan based on the combined results of morphology and molecular analyses. Male and female of the previously known species H. mogera (Yaginuma, 1972), N. brevipes (Yaginuma, 1970), and N. okinawaensis (Yaginuma, 1979) are redescribed and illustrated using type specimens or specimens collected at the type locality. Nesticella terrestris (Yaginuma, 1970) is resurrected as a valid species and distinguished from the closely related N. brevipes based on morphological and molecular evidence. The male of N. terrestris is described for the first time. We herein describe three new species of Nesticella and two new species of Howaia from different islands in the Ryukyu Archipelago, southwest Japan: N. insulanasp. nov. (♂, ♀) from Yonaguni-jima Island, N. occultasp. nov. (♀) from Ishigaki-jima Island, N. silvicolasp. nov. (♂, ♀) from Yakushima Island, H. albasp. nov. (♂, ♀) from Miyako-jima Island and H. subterraneasp. nov. (♀) from Okinoerabu-jima Island. Nesticella occultasp. nov., H. albasp. nov. and H. subterraneasp. nov. dwell exclusively in caves and show extensive morphological adaptation to subterranean life.
