The genome of Azoarcus olearius DQS‐4^T, a N₂‐fixing Betaproteobacterium isolated from oil‐contaminated soil in Taiwan, was sequenced and compared with other Azoarcus strains. The genome sequence showed high synteny with Azoarcus sp. BH72, a model endophytic diazotroph, but low synteny with five non‐plant‐associated strains (Azoarcus CIB, Azoarcus EBN1, Azoarcus KH32C, A. toluclasticus MF63^T and Azoarcus PA01). Average Nucleotide Identity (ANI) revealed that DQS‐4^T shares 98.98% identity with Azoarcus BH72, which should now be included in the species A. olearius. The genome of DQS‐4^T contained several genes related to plant colonization and plant growth promotion, such as nitrogen fixation, plant adhesion and root surface colonization. In accordance with the presence of these genes, DQS‐4^T colonized rice (Oryza sativa) and Setaria viridis, where it was observed within the intercellular spaces and aerenchyma mainly of the roots. Although they promote the growth of grasses, the mechanism(s) of plant growth promotion by A. olearius strains is unknown, as the genomes of DQS‐4^T and BH72 do not contain genes for indole acetic acid (IAA) synthesis nor phosphate solubilization. In spite of its original source, both the genome and behaviour of DQS‐4^T suggest that it has the capacity to be an endophytic, nitrogen‐fixing plant growth‐promoting bacterium.