A novel Gram-staining-negative, oval-shaped (0.4-0.6×0.8-1.0 µm), non-motile strain without flagella, designated B7T, was isolated from deep seawater in the South China Sea. Strain B7T was able to grow at 25-40 °C (optimum 35 °C), at pH 5.5-9.0 (optimum pH 7.0) and with 0-8 % (w/v) NaCl (optimum 3 %). Chemotaxonomic analysis showed that the predominant isoprenoid quinone was Q-10 and the dominant fatty acids were C19 : 0 cyclo 8c and summed feature 8 (C18 : 1ω7c/C18 : 1ω6c). The polar lipids of strain B7T were diphosphatidyglycerol, phosphatidylglycerol, phosphatidylethanolamine, phosphatidylcholine, one unknown aminophospholipid, one unknown glycolipid and three unknown lipids. The DNA G+C content of the genomic DNA was 65.1 mol%. Phylogenetic analysis of 16S rRNA gene sequences showed that strain B7T belongs to the genus Mesorhizobium with similarities ranging from 96.2 to 97.5 %. Phylogenetic analyses of housekeeping genes recA, atpD and glnII indicated that strain B7T represented a distinct evolutionary lineage with the genus Mesorhizobium. OrthoANI values between strain B7T and related strains of the genus Mesorhizobium (<80 %) were lower than the threshold value of 95 % ANI relatedness for species demarcation. Therefore, strain B7T is concluded to represent a novel species of the genus Mesorhizobium, for which the name Mesorhizobium oceanicumsp. nov. is proposed. The type strain is B7T (=KCTC 42783T=MCCC 1K02305T).