Azoarcus is a genus of nitrogen-fixing bacteria. Species in this genus are usually found in contaminated water, as they are involved in the degradation of some contaminants, commonly inhabiting soil. These bacteria have also been found growing in the endophytic compartment (inside the plant between the living cells) of some rice species and other grasses. The genus is within the family in the Rhodocyclales of the Betaproteobacteria.