Strain Eri-1T was isolated from a water sample on the campus of Kyushu University, Fukuoka, Japan. The motility and morphology of the isolate were similar to those of members of the genus Leptospira , but the spiral structure of the isolate was sharper under dark-field microscopy. Cells were 10.6±1.3 µm long and 0.2 µm in diameter, with a wavelength of 0.9 µm and an amplitude of 0.4 µm. Strain Eri-1T grew in Korthof’s medium at both 13 and 30 °C, and also in the presence of 8-azaguanine. 16S rRNA gene-based phylogenetic analysis placed strain Eri-1T within the radiation of the genus Leptospira where it formed a unique lineage within the clade of the known saprophytic species of the genus Leptospira . The strain was not pathogenic to hamsters. Strain Eri-1T exhibited low levels (11.2–12.6 %) of similarity by DNA–DNA hybridization to the three most closely related species of the genus Leptospira . The DNA G+C content of the genome of strain Eri-1T was 42.5±0.1 mol%. These results suggest that strain Eri-1T represents a novel species of the genus Leptospira , for which the name Leptospira idonii sp. nov. is proposed. The type strain is Eri-1T ( = DSM 26084T = JCM 18486T).