Steinernema taiwanensis is a recently identified entomopathogenic nematode (EPN) which was isolated from soil in Taiwan. In this study, we investigated an association between S. taiwanensis and its bacterial symbiont using a polyphasic taxonomic approach in this study. The strain TCT-1, which was isolated from S. taiwanensis, shared the highest sequence identity of the 16S rRNA gene to Xenorhabdus spp. but below the threshold of 98.7% required to recognize a new species of EPN bacterial symbionts. Multilocus sequence analysis showed that the strain TCT-1 formed a distinct sister clade to X. griffiniae, X. ehlersii and X. thuongxuanensis When the genome sequence of TCT-1 was compared with those of Xenorhabdus spp., the average nucleotide identity (ANI) scores and digital DNA-DNA hybridization (dDDH) values were below 95% and 70%, respectively. TCT-1 is closely related to X. ehlersii DSM 16337 with the dDDH value of 62.6% and the ANI score 94.15%. Phenotypical characterization also revealed that TCT-1 differed from other Xenorhabdus spp. in biochemical characteristics. Based on the polyphasic taxonomic results, we propose that the strain TCT-1T (= BCRC 81417T) represents a new species within Xenorhabdus, for which the name Xenorhabdus taiwanensissp. nov. is proposed.