Aerobic, Gram-stain-negative, obligately chemolithoautotrophic thiosulfate-oxidizing bacteria, strains AkT22T and aks77T were isolated from a brackish lake in Japan. Strains AkT22T and aks77T were isolated from samples of eelgrass and sediment, respectively. Growth on sulfide, tetrathionate, elemental sulfur, and organic substrates was not observed for both strains. Growth of the strains was observed at 5 °C or higher temperature, with optimum growth at 22 °C. Strain AkT22T grew at a pH range of 5.8-8.0, with optimum growth at pH 6.7-7.8. Strain aks77T grew at a pH range of 5.8-8.5, with optimum growth at pH 7.0-7.9. Major cellular fatty acids (> 10% of total) of strain AkT22T were C16:1, C18:1, and C16:0. The sole respiratory quinone was ubiquinone-8 in both strains. The genome of strain AkT22T consisted of a circular chromosome, with size of approximately 2.6 Mbp and G + C content of 43.2%. Those values of the genome of strain aks77T were ca. 2.7 Mbp and 45.5%, respectively. Among cultured bacteria, Thiomicrorhabdus aquaedulcis HaS4T showed the highest sequence identities of the 16S rRNA gene, to strains AkT22T (94%) and aks77T (95%). On the basis of these results, Thiosulfativibrio zosterae gen. nov., sp. nov. and Thiosulfatimonas sediminis gen. nov., sp. nov. are proposed, with type strains of AkT22T (= BCRC 81184T = NBRC 114012T = DSM 109948T) and aks77T (= BCRC 81183T = NBRC 114013T), respectively.