A novel bacterium (strain K4(T)) belonging to the genus Sphingomonas was isolated from tidal flat sediment in Korea. Its morphology, physiology, biochemical features and 16S rRNA gene sequence were characterized. Colonies of this strain are yellow in colour and the cells are rod-shaped, exhibiting negative Gram staining. The strain grows at 0-5 % (w/v) NaCl and 20-35 degrees C, with optimal growth occurring at 0 % (w/v) NaCl and 30 degrees C. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain K4(T) is associated with the genus Sphingomonas. Within the phylogenetic tree, this novel strain shares a branching point with Sphingomonas asaccharolytica Y-345(T), with which it shares 97.3 % 16S rRNA gene sequence similarity. The polyamine pattern predominantly contains the Sphingomonas-specific triamine sym-homospermidine. Combined analysis of 16S rRNA gene sequences, DNA-DNA relatedness, physiological and biochemical test results identified genotypic and phenotypic differences between strain K4(T) and other Sphingomonas species. On the basis of these differentiating features, it is concluded that strain K4(T) (=KCTC 22050(T)=DSM 19475(T)) represents a novel species of the genus Sphingomonas, for which the name Sphingomonas aestuarii sp. nov. is proposed.