Azotobacter is a genus of large (2 micrometers or more in diameter), rod-shaped or oval, aerobic, free-living Gram negative bacteria that live in the soil. There are about six different species of Azotobacter.

The bacteria can fix atmospheric nitrogen by turning it into ammonia if they have an energy source (often a carbohydrate, but they can also use alcohols and organic acids) available. Thus, these bacteria are important in agriculture because nitrogen needs to be fixed before it is usable to many crop plants.

Azotobacter vinlandii is commonly used in genetics and cellular biology research.