Finding male philometrid nematodes is essential for taxonomic identification among congeneric species. In this study, male Philometra thaiensis Moravec, Fiala et Dyková, 2004 were collected and described for the first time, from the body cavity of the freshwater fish (eyespot pufferfish) Tetraodon biocellatus Tirant (Tetraodontiformes, Tetraodontidae), and conspecific females were redescribed based on the additional morphological biometrics examined. Molecular examination was carried out on the small subunit 18S rRNA, revealing the evolutionary relationships of P. thaiensis and reported philometrid species (Philometra and Philometroides) from Japan with other dracunculoids deposited in the GenBank. Based on the molecular data, there are some genera (Philometra, Philometroides, Clavinema, and Margolisianum [genus inquirendum]) requiring further morphological re-evaluation that should be supported with molecular data.