A Gram-negative, catalase- and oxidase-positive, non-sporulating, rod-shaped and slightly halophilic bacterial strain, designated UST090418-1611(T), was isolated from the marine sponge Xestospongia testudinaria collected from the Red Sea coast of Saudi Arabia. Phylogenetic trees based on the 16S rRNA gene sequence placed strain UST090418-1611(T) in the family Alteromonadaceae with the closest relationship to the genus Marinobacter. The 16S rRNA gene sequence similarity between the strain and the type strains of recognized Marinobacter species ranged from 92.9 to 98.3%. Although strain UST090418-1611(T) shared high 16S rRNA gene sequence similarity with Marinobacter mobilis CN46(T), M. zhejiangensis CN74(T) and M. sediminum R65(T) (98.3, 97.4 and 97.3%, respectively), the relatedness of the strain to these three strains in DNA-DNA hybridization was only 58, 56 and 33%, respectively, supporting the novelty of the strain. In contrast to most strains in the genus Marinobacter, strain UST090418-1611(T) tolerated only 6% (w/v) NaCl, and optimal growth occurred at 2.0% (w/v) NaCl, pH 7.0-8.0 and 28-36 °C. The predominant cellular fatty acids were C(12:0) 3-OH, C(16:0), C(12:0) and summed feature 3 (C(16:1)ω6c and/or C(16:1)ω7c). The genomic DNA G+C content was 57.1 mol%. Based on the physiological, phylogenetic and chemotaxonomic characteristics presented in this study, we suggest that the strain represents a novel species in the genus Marinobacter, for which the name Marinobacter xestospongiae sp. nov. is proposed, with UST090418-1611(T) ( = JCM 17469(T) = NRRL B-59512(T)) as the type strain.