An aerobic, Gram-negative bacterial strain, TH-G33T, was isolated from freshwater sediment of Taihu Lake in China. The taxonomy of strain TH-G33T was studied by using phenotypic and phylogenetic methods. Cells of strain TH-G33T were coccoid rods or rods and formed colourless to pale-pink colonies on nutrient agar. Phylogenetic analysis based on nearly complete 16S rRNA gene sequences showed that strain TH-G33T was related to Roseomonas mucosa (94.4 %), Roseomonas gilardii subsp. gilardii (94.1 %), Roseomonas gilardii subsp. rosea (94.8 %) and Roseomonas cervicalis (93.9 %). Cells contained ubiquinone 10 (Q-10) as the major quinone and the G+C content was 71.9 mol%. Thus, strain TH-G33T represents a novel species of the genus Roseomonas, for which the name Roseomonas lacus sp. nov. is proposed. The type strain is TH-G33T (= CGMCC 1.3617T = JCM 13283T).