Sphaerularia vespae sp. nov., an endoparasite of a common Japanese hornet, Vespa simillima is described from Hokkaido, Japan, and its molecular sequence profiles are given. This newly discovered nematode appears to belong to the genus Sphaerularia, judging from its characteristic parasitic form, the uterium, which looks like a sausage with many verrucae on its surface. Sphaerularia vespae sp. nov. is distinguished from S. bombi, the only other nominal species of the genus, by the morphology of the male bursa, female tail, and anterior end of parasitic juveniles. SSU, ITS, and D2D3 (LSU) DNA sequences were determined for S. vespae sp. nov. and compared with those from related nematodes obtained from the GenBank database. The sequences from S. vespae sp. nov. were close to those of S. bombi and several another tylenchid entomoparasitic nematodes. Sphaerularia vespae sp. nov. parasitizes overwintering gynes of the hornet and practically sterilizes them, as S. bombi does for bumblebee gynes.