A novel obligately anaerobic spirochete strain K2^T was isolated from bottom marine sediments at Crater Bay of Yankicha Island (Kuril Islands, Russia). Strain K2^T had helical shape and Gram-negatively stained. The optimal growth conditions were as follows: the optimum temperature was 28–30 °C with range 5–34 °C; optimal pH at 7.0–7.5 with range of 6.8–8.5; NaCl optimum at 3–3.5 % (w/v) and range of 1–7 % (w/v). Strain K2^T was catalase- and oxidase-negative. Glucose fermentation products were acetate, lactate, ethanol, CO₂, H₂. The major fatty acids were C_{14 : 0}, iso-C_{13 : 0}, iso-C_15:0, C_{14 : 0} DMA, iso-C_{15 : 0} DMA. The G+C content of genomic DNA was 43.2 mol%. Phylogenetic analyses of 16S rRNA genes showed that strain K2^T belonged to the genus Oceanispirochaeta of the family Spirochaetaceae . The 16S rRNA gene sequence similarity of strain K2^T and O. litoralis DSM 2029^T and O. sediminicola DSM 104770^T was 96 and 94 %, respectively. Based on the results of our study, we propose the name Oceanispirochaeta crateris sp. nov.; type strain K2^T (=DSM 16308^T=VKM B-3266^T). Also, the taxonomic status of Spirochaeta perfilevii was revised: 16S rRNA genes sequence showed less than 89 % similarity to nearest phylogenetic neighbours. Therefore, we proposed to separate this species into a novel genus Thiospirochaeta - T. perfilievii gen. nov., comb. nov.