A Gram-stain-negative and facultatively anaerobic bacterium, SY21(T), was isolated from marine sediments of the coastal area in Weihai, China (122° 0' 37" E 37° 31' 33" N). Cells of strain SY21(T) were 0.3-0.5 μm wide and 1.5-2.5 μm long, catalase- and oxidase-positive. Colonies on 2216E agar were transparent, beige- to pale-brown-pigmented, and approximately 0.5 mm in diameter. Growth occurred optimally at 33-37 °C, pH 7.0-7.5 and in the presence of 2-3% (w/v) NaCl. Phylogenetic analysis of the 16S rRNA gene indicated that strain SY21(T) was a member of the genus Mariniphaga within the family Prolixibacteraceae. The closest described neighbour in terms of 16S rRNA gene sequences identity was Mariniphaga anaerophila Fu11-5(T) (94.7%). The major respiratory quinone of strain SY21(T) was MK-7, and the dominant fatty acids were iso-C15 : 0, iso-C17 : 0 3-OH and anteiso-C15 : 0. The major polar lipids were phosphatidylethanolamine, aminolipid and an unidentified lipid, and the DNA G+C content was 37.9 mol%. The distinct phylogenetic position and phenotypic traits distinguished the novel isolate from M. anaerophila Fu11-5(T). Phenotypic and genotypic analysis indicated that strain SY21(T) could be assigned to the genus Mariniphaga. The name Mariniphaga sediminis sp. nov. is proposed, with the type strain SY21(T) ( = KCTC 42260(T) = MCCC 1H00107(T)).