A novel Gram-stain positive, endospore-forming bacterium, designated SA4T, was isolated from a soil sample collected from an abandoned marine solar saltern at Wendeng, Shandong Province, PR China. Cells were observed to be rod shaped, alginase positive, catalase positive and motile. The strain was found to grow at temperatures ranging from 15 to 40 °C (optimum 35 °C), and pH 5.0–11.0 (optimum pH 8.0) with 0–7.0 % (w/v) NaCl concentration (optimum NaCl 3.0 %). Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain SA4T belongs to the genus Bacillus and exhibits 16S rRNA gene sequence similarities of 96.6, 96.5, 96.3 and 96.2 % with Bacillus horikoshii DSM 8719T, Bacillus acidicola 105-2T, Bacillus shackletonii LMG 18435T and Bacillus pocheonensis Gsoil 420T, respectively. The menaquinone was identified as MK-7 and the major polar lipids were identified as diphosphatidylglycerol, phosphatidylglycerol and phosphatidylethanolamine. The major fatty acids detected were anteiso-C15:0 (22.3 %), iso-C15:0 (22.6 %), iso-C16:0 (14.8 %) and iso-C14:0 (14.7 %). The DNA G+C content was determined to be 42.4 mol %. Phenotypic, chemotaxonomic and genotypic properties clearly indicated that isolate SA4T represents a novel species within the genus Bacillus, for which the name Bacillus mesophius sp. nov. is proposed. The type strain is SA4T (=DSM 101000T=CCTCC AB 2015209T).