A novel mesophilic chemoheterotrophic bacterial strain designated LMB275T, that grows under aerobic/microaerophilic conditions, was isolated from the thermal spring water of a deep aquifer in France. Cells were oxidase-positive and catalase-negative, with a polar flagellum. Phylogenetic analyses based on 16S rRNA gene sequences suggested that strain LMB275T belongs to the genus Aquabacterium, and most closely related to Aquabacterium parvum B6T (99.1 %), Aquabacterium commune B8T (97.9 %) and Aquabacterium citratiphilum B4T (97.0 %). The DNA-DNA relatedness between strain LMB275T and A. parvum was below 70.0 %. The DNA G+C content was 66 mol %. The fatty acid composition differed from all other species of the genus with the dominance of C18 :1 9c. The polar lipids consisted of phosphatidylethanolamine as the major component, phosphatidylglycerol, diphosphatidylglycerol and several uncharacterized phospholipids. The strain was not able to assimilate all the carbohydrates tested and used different inorganic electron acceptors. Based on the physiological, chemotaxonomic and phylogenetic analyses, and the low level of DNA relatedness with the closest phylogenetic relative A. parvum, it can be concluded that the strain LMB275T represents a new species of the genus Aquabacterium, for which the name Aquabacterium silicae sp. nov. is proposed. The type strain of A. silicae is LMB275T (CNCM I-5858).