A heterotrophic, Gram-stain-negative, aerobic, sodium-requiring and motile bacterium was isolated from oil-contaminated surface water of the Gulf of Mexico during the Deepwater Horizon oil spill. Strain O3.65T showed highest 16S rRNA gene sequence similarity to Phaeobacter gallaeciensis BS107T and Phaeobacter inhibens T5T, both with 98.3 %, respectively. Based on complete genome analysis, highest similarity was observed to species of the genus Ruegeria. Strain O3.65T exhibited a broad salinity, temperature and pH range of 0.5-10 % NaCl, 4-45 °C and 5.5-9.0, respectively. The DNA G+C content of strain O3.65T was 61.5 mol%. The major respiratory lipoquinone was ubiquinone-10 (Q-10), the most dominant fatty acids (>1 %) comprised 18 : 1ω7c and 18 : 1ω7c 11-methyl, 10 : 0 3OH, 12 : 1 3OH, 14 : 1 3OH/3-oxo-14 : 0, 16 : 0, 16 : 0 2OH, 18 : 1 2OH and 12 : 1. The polar lipid pattern indicated presence of phosphatidylcholine, phosphatidylglycerol, an unidentified aminolipid, two unidentified phospholipids and seven unidentified lipids. On Difco marine broth agar, strain O3.65T formed smooth, shiny white to beige and convex colonies with regular edges. Phylogenetic, phylogenomic and phenotypic differences revealed that strain O3.65T represents a new species of a novel genus within the family Rhodobacteraceae, for which we propose the name Tritonibacter horizontis gen. nov., sp. nov. The type strain of the type species is O3.65T (=DSM 101689T=LMG 29740T).