A Gram-negative, long rod-shaped, and yellowish bacterium, designated as strain 15J17T(T), was isolated from sediment of the Han River in South Korea after exposure to 3 kGy of gamma radiation. The strain was catalase- and oxidase-positive and showed resistance to gamma radiation-D10 value (i.e., the dose required to reduce the bacterial population by 10-fold) of >4 kGy. Phylogenetic analysis based on the 16S rRNA gene sequence revealed that the strain belonged to the genus Spirosoma and showed moderate degrees of sequence similarity with related species (90.6-93.5 %). Chemotaxonomic data revealed that the strain contained summed feature 3 (C16:1 ω7c/C16:1 ω6c), C16:1 ω5c, C16:0, C18:0, and C15:0 iso as the major fatty acids; phosphatidylethanolamine, an unidentified aminophospholipid, and an unidentified polar lipid as the major polar lipids; and menaquinone-7 (MK-7) as the major quinone. The genomic DNA G+C content of the new strain was 48.3 mol%. Based on these data, type strain 15J17T(T) (=KCTC 52198(T) = JCM 31409(T)) should be classified as representing a new species, for which we propose the name Spirosoma fluminis sp. nov.