The taxonomic status of an actinobacterium that changed colour during growth, strain 42H12-1(T), isolated from deep-sea sediment collected from the Atlantic Ocean, was established using a combination of genotypic and phenotypic data. Strain 42H12-1(T) formed a distinct branch in the 16S rRNA gene phylogenetic tree together with the type strains in the genus Modestobacter. The highest sequence similarity by blast analysis was to Modestobacter versicolor CP153-2(T) (98.5 %) and the second-highest sequence similarity was to Modestobacter multiseptatus AA-826(T) (97.5 %). DNA-DNA relatedness of only 12 % (sd 1.82 %) between strain 42H12-1(T) and M. versicolor DSM 16678(T) differentiated them as members of separate genomic species. Colonies of strain 42H12-1(T) were black on oligotrophic medium, but orange to red, turning black, on copiotrophic medium. The peptidoglycan contained meso-diaminopimelic acid. The polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol and an unknown aminophospholipid. The predominant menaquinone was MK-9(H(4)). The major fatty acids were iso-C(16 : 0) and C(17 : 1)ω8c. The DNA G+C content was 72.3±1 mol%. Strain 42H12-1(T) ( = DSM 45201(T) = CGMCC 4.5581(T)) is assigned as the type strain of a novel species of the genus Modestobacter, for which the name Modestobacter marinus sp. nov. is proposed.