A novel Gelidibacter strain, JCM 31967T, was isolated from seawater collected from the Inland Sea (Setonaikai) in Japan. It was characterized as a Gram-negative, halophilic, oxidase-negative, catalase-positive, aerobic, nonmotile, but gliding, rod-shaped bacterium without flagella. Based on 16S rDNA gene identity, strain JCM 31967T is closely related to Gelidibacter mesophilus (DSM 14095T, 96.6% identity), G. gilvus (IC158T, 96.4%), G. algens (DSM 12408T, 96.1%), G. sediminis (S11-41T, 94.7%), and G. salicanalis (IC162T, 94.7%). The G + C content of strain JCM 31967T DNA was found to be 39.1%. The average nucleotide identity (ANI) values of JCM 31967T against G. algens DSM 12408T and G. mesophilus DSM 14095T were 79.1% and 80.9%, respectively. Strain JCM 31967T phenotypically differed from the closest related Gelidibacter species in its utilization of methyl α-d-mannopyranoside, methyl α-d-glucopyranoside, and d-ribose and in its lack of utilization of l-arginine and d-arabinose. It was further differentiated based on its fatty acid composition, specifically properties of C18:0 and C20:2 ω6c, 9c, which were significantly different from those of G. algens, G. gilvus, G. mesophilus, G. salicanalis, and G. sediminis type strains. Overall, the results of DNA–DNA hybridization and physiological and biochemical analyses differentiated strain JCM 31967T from a previously described species of Gelidibacter. Based on these polyphasic taxonomic findings, it was concluded that strain JCM 31967T is a novel Gelidibacter species, for which the name Gelidibacter japonicus sp. nov. is proposed, with JCM 31967T (= LMG 30063T) as the type strain.