<jats:p>A Gram-stain-negative, aerobic, motile and rod-shaped strain, THG-DN7.3<jats:sup>T</jats:sup>, was isolated from a waterfall. Strain THG-DN7.3<jats:sup>T</jats:sup> grew well at 18–28 °C and at pH 6.0–7.5 on Reasoner's 2A agar. Based on 16S rRNA gene sequence comparisons, strain THG-DN7.3<jats:sup>T</jats:sup> was most closely related to <jats:italic>Undibacterium jejuense</jats:italic> JS4-4<jats:sup>T</jats:sup> (97.3 % 16S rRNA gene sequence similarity) and <jats:italic>Undibacterium seohonense</jats:italic> SHS5-24<jats:sup>T</jats:sup> (96.5 %). The G+C content of the genomic DNA was 57.4 mol%. The mean DNA–DNA relatedness of strain THG-DN7.3<jats:sup>T</jats:sup> with <jats:italic>U. jejuense</jats:italic> KACC 12607<jats:sup>T</jats:sup> was 40 ± 1 % (reciprocal 50 ± 2.1 %). The major cellular fatty acids of strain THG-DN7.3<jats:sup>T</jats:sup> were summed feature 3 (C<jats:sub>16 : 1</jats:sub>ω7<jats:italic>c</jats:italic> and/or C<jats:sub>16 : 1</jats:sub>ω6<jats:italic>c</jats:italic>) (47.4 %), C<jats:sub>16 : 0</jats:sub> (30.4 %), summed feature 8 (C<jats:sub>18 : 1</jats:sub>ω6<jats:italic>c</jats:italic> and/or C<jats:sub>18 : 1</jats:sub>ω7<jats:italic>c</jats:italic>) (6.8 %) and C<jats:sub>12 : 0</jats:sub> (6.2 %). The predominant isoprenoid quinone was ubiquinone-8. The major polar lipids were phosphatidylethanolamine, phosphatidylglycerol and diphosphatidylglycerol. The results of the DNA–DNA hybridization and genotypic analysis, in combination with chemotaxonomic and physiological data, demonstrated that strain THG-DN7.3<jats:sup>T</jats:sup> represents a novel species of the genus <jats:italic>Undibacterium</jats:italic>, for which the name <jats:italic>Undibacterium aquatile</jats:italic> sp. nov. is proposed. The type strain is THG-DN7.3<jats:sup>T</jats:sup> ( = KCTC 42243<jats:sup>T</jats:sup> = CCTCC AB 2015119<jats:sup>T</jats:sup>).</jats:p>