An aerobic, Gram-stain-negative, non-pigmented, non-motile bacterium, strain KMM 9031(T), was isolated from a sandy sediment sample collected from the shore of the Sea of Japan and subjected to phenotypic and phylogenetic analysis. Based on comparative 16S rRNA gene sequence analysis, strain KMM 9031(T) constituted a separate phylogenetic line within the Roseobacter clade of the class Alphaproteobacteria, sharing highest sequence similarities with members of the genera Roseovarius (92.7-95.3 %), Pseudoruegeria (94.5 %), Sulfitobacter (92.7-94.4 %) and Thalassobacter (94.2-94.3 %). The predominant fatty acid of strain KMM 9031(T) was C(18 : 1)ω7c, with C(16 : 0), C(10 : 0) 3-OH and C(12 : 1) 3-OH present in lesser amounts. The DNA G+C content of the isolate was 52.6 mol%. The major isoprenoid quinone was Q-10 and polar lipids comprised phosphatidylcholine, phosphatidylglycerol, diphosphatidylglycerol and two unknown lipids. On the basis of phylogenetic analysis and physiological and biochemical characterization, strain KMM 9031(T) represents a novel species in a new genus, for which the name Pacificibacter maritimus gen. nov., sp. nov. is proposed; the type strain is KMM 9031(T) ( = NRIC 0785(T) = JCM 17096(T)).