A strictly aerobic, Gram-stain-negative, ovoid-shaped and non-motile bacterium, designated as strain MJ-SS7T, was isolated from tidal mudflat sediment collected at a macro-tidal clam farm in Gochang, Republic of Korea. Strain MJ-SS7T exhibited oxidase- and catalase-positive activities. Growth occurred at 20–37 °C (optimum: 30 °C), pH 6–8 (optimum: pH 7) and in the presence of 0–9% (w/v) NaCl (optimum: 3%). Phylogenetic analysis based on 16S rRNA gene sequences revealed that MJ-SS7T belonged to the family Roseobacteraceae. Its closest relatives, based on sequence similarity, were Rhodophyticola porphyridii KACC 18805T (95.9%), Kangsaoukella pontilimi KCTC 72224T (95.8%) and Actibacterium pelagium JN33T (95.5%). The major fatty acids included summed feature 8 (C18:1
ω7c/C18:1
ω6c), C16:0, C16:0 2OH and C18:0. The major polar lipids consisted of phosphatidylglycerol, five unidentified phospholipids, four unidentified aminolipids, two unidentified aminophospholipids and four unidentified lipids. The major respiratory quinone was ubiquinone-10. The genome size was 5.20 Mb, with a DNA G+C content of 63.4 mol%, encoding 5,031 protein-coding genes, 3 rRNA genes and 46 tRNA genes. Comparative genomic analyses confirmed its novelty, with average nucleotide identity (<73%) and average amino acid identity (<70%) values below the species delineation thresholds. Based on polyphasic taxonomic data, strain MJ-SS7T represents a novel species of a new genus within the family Roseobacteraceae, for which the name Ovoidimarina sediminis gen. nov., sp. nov. is proposed. The type strain is MJ-SS7T (=KEMB 21436T=KCTC 8329T=JCM 36750T).