A novel thermophilic and sulfur-reducing bacterium, strain S304(T), was isolated from the Taketomi submarine hot spring shallow hydrothermal field located at southern part of the Yaeyama Archipelago, Japan. The cells were non-motile short thick rods or oval cocci 1.1-2.7 μm in length and 1.1-1.9 μm in width. Strain S304(T) was an obligately anaerobic heterotroph and sulfur reduction stimulates growth. Growth was observed between 50-65°C (optimum 60°C), pH 6.2-8.0 (optimum pH 7.1), 1.0-6.0% NaCl concentration (optimum 3.0%). The fatty acid composition was C(16:0) (71.4%), C(18:0) (20.9%) and C(18:1) (7.7%). The G + C content of genomic DNA was 40.8 mol%. The 16S rRNA gene sequence analysis indicated that strain S304(T) belonged to the genus Kosmotoga. Based on physiological and phylogenetic features of a new isolate, we propose new species in the genus Kosmotoga: the type strain of Kosmotoga arenicorallina sp. nov is S304(T) (=JCM 15790(T) = DSM22549(T)). Thermococcoides shengliensis 2SM-2(T) is phylogenetically associated with Kosmotoga olearia 14.5.1(T). Based on the phylogenetic relationship between Thermococcoides shengliensis 2SM-2(T) and Kosmotoga olearia 14.5.1(T), we propose the reclassification of Thermococcoides shengliensis as Kosmotoga shengliensis comb. nov. (type strain 2SM-2(T)). In addition, an emended description of the genus Kosmotoga is proposed.