A novel bacterium B9(T) was isolated from tidal flat sediment. Its morphology, physiology, biochemical features, and 16S rRNA gene sequence were characterized. Colonies of this strain are yellow and the cells are Gram-negative, rod-shaped, and do not require NaCl for growth. The 16S rRNA gene sequence similarity indicated that strain B9(T) is associated with the genus Lysobacter (< or = 97.2%), Xanthomonas (< or = 96.8%), Pseudomonas (< or = 96.7%), and Luteimonas (< or = 96.0%). However, within the phylogenetic tree, this novel strain shares a branching point with the species Luteimonas composti CC-YY255(T) (96.0%). The DNA-DNA hybridization experiments showed a DNA-DNA homology of 23.0% between strain B9(T) and Luteimonas mephitis B1953/27.1(T). The G+C content of genomic DNA of the type strain is 64.7 mol% (SD, 1.1). The predominant fatty acids are iso-C(11:0), iso-C(15:0), iso-C(16:0), iso-C(17:0), iso-C(17:0) omega9c, and iso-C(11:0) 3-OH. Combined analysis of the 16S rRNA gene sequences, fatty acid profile, and results from physiological and biochemical tests indicated that there is genotypic and phenotypic differentiation of the isolate from other Luteimonas species. For these reasons, strain B9(T) was proposed as a novel species, named Luteimonas aestuarii. The type strain of the new species is B9(T) (= KCTC 22048(T), DSM 19680(T)).