A novel obligately anaerobic, extremely thermophilic, organotrophic bacterium, strain 1445tT, was isolated from a hot spring on Kunashir Island (Kuril Islands, Russia). Cells were motile rods (0.4–0.5×1.0–3.0 µm). The temperature range for growth at pH 7.8 was 46–80 °C, with optimum growth at 65 °C. The pH range for growth at 65 °C was pH 5.7–9.0, with optimum growth at pH 7.8. Growth was not observed at or below 40 °C, at or above 84 °C, at or below pH 5.4 or at or above pH 9.5. The isolate degraded a wide range of substrates including starch, cellulose and cellulose derivatives. Elemental sulfur stimulated growth, but sodium sulfate, sulfite and thiosulfate did not. DNA G+C content was 31 mol%. Phylogenetic analysis of 16S rRNA gene sequences showed that strain 1445tT belonged to the genus Fervidobacterium. 16S rRNA gene sequence similarities with strains of other species of the genus Fervidobacterium were 94.9–98.3 %; the type strain of Fervidobacterium gondwanense was the closest relative of strain 1445tT. DNA–DNA hybridization of strain 1445tT and F. gondwanense AB39T revealed a relatedness value of 20 %. Based on phylogenetic data and physiological properties of the isolate, a novel species, designated Fervidobacterium riparium sp. nov., is proposed with strain 1445tT ( = DSM 21630T = VKM B-2549T) as the type strain.