A moderately thermophilic and strictly anaerobic bacterium, designated HS1(T), was isolated from offshore hot spring sediment in Xiamen, China. Cells were Gram-negative, catalase-positive, oxidase-negative, slender and flexible rods without flagella. The strain could grow at 35-55 °C (optimum at 50 °C) and in 1-8 % NaCl (w/v; optimum 2-4 %). Phylogenetic analysis based on 16S rRNA gene sequences showed that strain HS1(T) was affiliated with the family Marinilabiliaceae and shared a distant relationship with the previously described genera. The isolate was most closely related to Anaerophaga thermohalophila Fru22(T) with 16S rRNA gene sequence similarity of 92.4 %, followed by the other members of the family Marinilabiliaceae with 88.7-91.1 % similarity. The dominant cellular fatty acids were iso-C(15 : 0) and anteiso-C(15 : 0). The predominant quinone was MK-7. The major polar lipids were phosphatidylethanolamine (PE) and an unknown polar lipid. The genomic DNA G+C content was 38.7 mol%. Besides the phylogenetically distant relationship, strain HS1(T) was obviously distinguished from the most closely related genera in several phenotypic properties including colony colour and pigment production, optimal temperature, optimal NaCl, relation to O(2), bicarbonate/carbonate requirement, catalase activity, nitrate reduction, fermentation products and cellular fatty acid profile. Based on the phenotypic and phylogenetic data, strain HS1(T) represents a novel species of a new genus, for which the name Thermophagus xiamenensis gen. nov., sp. nov. is proposed. The type strain of the type species is HS1(T) (= DSM 19012(T) = CGMCCC 1.5071(T)).