A Gram-stain-negative bacterial strain designated Con4T was isolated from activated sludge in a seawater treatment system. The strain was rod-shaped, motile, aerobic, and formed yellow-colored colonies on agar medium. Cells contained carotenoid pigments, but flexirubin-type pigments were absent. Based on 16S rRNA gene sequence analysis, the strain was assigned to the genus Gelidibacter. Optimum growth occurred at 20 °C, pH 7.0, and 1-2% (w/v) NaCl. Prominent fatty acid types were iso-C15:0, anteiso-C15:0, and iso-C16:0 3OH. Diphosphatidylglycerol and phosphatidylethanolamine were present as the major polar lipids. MK-6 was present as major menaquinone. Strain Con4T showed the highest sequence similarity with Gelidibacter mesophilus KCTC 12106T (96.5%), Gelidibacter gilvus IC158T (96.4%), and Gelidibacter algens ACAM 536T (95.8%). The G+C mol% contents of the strain Con4T were 37.7%. Distinct morphological, physiological, and genotypic differences from the previously described taxa support the classification of strain Con4T as a representative of a novel species in the genus Gelidibacter, for which the name Gelidibacter flavus sp. nov. is proposed. The type strain is Con4T (=KEMB 41-198T = JCM 31135T).