A Gram-stain-negative, non-motile, non-spore-forming, rod-shaped, aerobic bacterium, designated 15J8-12T, was isolated from a water sample after exposure to 3 kGy of gamma radiation. The strain showed resistance to gamma radiation with a dose required to reduce the bacterial population 10 fold (D10) value of 4.7 kGy. The results of comparative 16S rRNA gene sequence analysis indicated that strain 15J8-12T represented a member of the family Cytophagaceae, phylum Bacteroidetes, and was most closely related to 'Spirosomafluminis' 15J17 (97.92 %) and Spirosoma arcticum R2-35T (92.22 %). The G+C content of the genomic DNA of 15J8-12T was 51.3 mol%. The detection of menaquinone MK-7 as the predominant respiratory quinone, a fatty acid profile with summed feature 3 (C16 : 1ω7c/C16 : 1ω6c; 40.5 %), C16 : 1ω5c (35.3 %), C15 : 0 iso (6.9 %) and C16 : 0 (6.8 %) as the major components and phosphatidylethanolamine as the major polar lipid also supported the affiliation of 15J8-12T with the genus Spirosoma. The DNA-DNA relatedness between 15J8-12T and 'Spirosoma fluminis' 15J17 was 27.8 %. On the basis of its phenotypic and genotypic properties, together with its phylogenetic distinctiveness, 15J8-12T should be considered to be a representative of a novel species of the genus Spirosoma, for which the name Spirosoma knui sp. nov. is proposed. The type strain is 15J8-12T (=KCTC 52510T=JCM 31407T).