A non-motile, rod-shaped bacterium, designated strain A5(T), was isolated from a dye wastewater treatment facility in the Republic of Korea. Cells were Gram-reaction-negative, aerobic, catalase-positive and oxidase-negative. The major fatty acids were C(16 : 0), C(18 : 0), iso-C(15 : 0) and summed feature 3 (C16 : 1)�s6c and/or C(16 : 1�s7c) and the major polar lipid was phosphatidylethanolamine. The strain contained MK-7 as the only isoprenoid quinone and the DNA G+C content was 41.2 mol%. A phylogenetic tree based on 16S rRNA gene sequences showed that strain A5(T) forms an evolutionary lineage within the radiation encompassing the members of genus Mucilaginibacter, with Mucilaginibacter lutimaris BR-3(T) as its nearest neighbour (96.6% sequence similarity). A number of phenotypic characteristics distinguished strain A5(T) from the members of the genus Mucilaginibacter. On the basis of the evidence presented in this study, strain A5(T) represents a novel species, for which the name Mucilaginibacter defluvii sp. nov. is proposed. The type strain is A5(T) (= KCTC 23922(T) = JCM 18283(T)).