Methane (CH 4 ) is a potent greenhouse gas that is released from fossil fuels and is also produced by microbial activity, with at least one billion tonnes of CH 4 being formed and consumed by microorganisms in a single year 1 . Complex methanogenesis pathways used by archaea are the main route for bioconversion of carbon dioxide (CO 2 ) to CH 4 in nature 2-4 . Here, we report that wild-type iron-iron (Fe-only) nitrogenase from the bacterium Rhodopseudomonas palustris reduces CO 2 simultaneously with nitrogen gas (N 2 ) and protons to yield CH 4 , ammonia (NH 3 ) and hydrogen gas (H 2 ) in a single enzymatic step. The amount of CH 4 produced by purified Fe-only nitrogenase was low compared to its other products, but CH 4 production by this enzyme in R. palustris was sufficient to support the growth of an obligate CH 4 -utilizing Methylomonas strain when the two microorganisms were grown in co-culture, with oxygen (O 2 ) added at intervals. Other nitrogen-fixing bacteria that we tested also formed CH 4 when expressing Fe-only nitrogenase, suggesting that this is a general property of this enzyme. The genomes of 9% of diverse nitrogen-fixing microorganisms from a range of environments encode Fe-only nitrogenase. Our data suggest that active Fe-only nitrogenase, present in diverse microorganisms, contributes CH 4 that could shape microbial community interactions.