The novel Gram-negative, aerobic, non-motile, non-spore-forming, short-rod bacterium, strain C7T, was isolated from a seawater sample from Menai Straits (Wales, UK) and characterized. Phylogenetic analysis of 16S rRNA gene sequences showed that this strain represented a distinct lineage within the Roseobacter clade of family Rhodobacteracea within Alphaproteobacteria . The members of the genera Pontivivens ( Pontivivens insulae GYSW-23T), Celeribacter ( Celeribacter manganoxidans DY2-5T), Donghicola ( Donghicola eburneus SW-277T), Roseovarius ( Roseovarius halotolerans HJ50T and Roseovarius pacificus 81-2T), Cribrihabitans ( Cribrihabitans marinus CZ-AM5T) and Aestuariihabitans ( Aestuariihabitans beolgyonensis BB-MW15T) were the closest relatives with 16S rRNA gene sequence identities between 93.4 and 95.6 %. Strain C7T could utilize a restricted number of complex substrates with a preference for yeast extract and tryptone, which is consistent with earlier observations that peptides may serve as an important energy and carbon source for bacteria from the Roseobacter clade. Growth occurred in the absence of sodium ions. The isolate C7T is a mesophilic bacterium that optimally grows at 20 °C. The strain can grow under microaerophilic conditions. The major fatty acid was C18 : 1 cis d11. The only detected ubiquinone was Q10. The polar lipids of strain C7T were phosphatidylglycerol, two unknown aminolipids and three unknown lipids. The DNA G+C content of the strain was 60.0 mol%. Based on the results of the morphological, physiological and phylogenetic analyses, the new genus, Monaibacterium gen. nov., to include the new species Monaibacterium marinum sp. nov., is proposed. Strain C7T (=DSM 100241T, =LMG 28800T) is the type and only strain of M. marinum.