Novel thermophilic heterotrophic bacteria were isolated from the subsurface of the volcanic island Surtsey off the south coast of Iceland. The strains were isolated from tephra core and borehole fluid samples collected below 70 m depth. The Gram-negative bacteria were rod-shaped (0.3–0.4 µm wide, 1.5–7 µm long), aerobic, non-sporulating and non-motile. Optimal growth was observed at 70 °C, at pH 7–7.5 and with 1% NaCl. Phylogenetic analysis identified the strains as members of the genus Rhodothermus. The type strain, ISCAR-7401 T, was genetically distinct from its closest relatives Rhodothermus marinus DSM 4252 T and Rhodothermus profundi PRI 2902 T based on 16S rRNA gene sequence similarity (95.81 and 96.01%, respectively), genomic average nucleotide identity (73.73 and 72.61%, respectively) and digital DNA–DNA hybridization (17.6 and 16.9%, respectively). The major fatty acids of ISCAR-7401 T were iso-C 17:0, anteiso-C 15:0, anteiso-C 17:0 and iso-C 15:0 (>10 %). The major isoprenoid quinone was MK-7 while phosphatidylethanolamine, diphosphatidylglycerol, an unidentified aminophospholipid and a phospholipid were the predominant polar lipid components. Based on comparative chemotaxonomic, genomic and phylogenetic analyses, we propose that the isolated strain represents a novel species of the genus Rhodothermus with the name Rhodothermus bifroesti sp. Nov. The type strain is ISCAR-7401 T (=DSM 112103 T =CIP 111906 T).