Brown adipose tissue (BAT) is specialized in energy expenditure, making it a potential target for anti-obesity therapies^,,,,. Following exposure to cold, BAT is activated by the sympathetic nervous system with concomitant release of catecholamines and activation of β-adrenergic receptors^,,,,. Because BAT therapies based on cold exposure or β-adrenergic agonists are clinically not feasible, alternative strategies must be explored. Purinergic co-transmission might be involved in sympathetic control of BAT and previous studies reported inhibitory effects of the purinergic transmitter adenosine in BAT from hamster or rat^,,. However, the role of adenosine in human BAT is unknown. Here we show that adenosine activates human and murine brown adipocytes at low nanomolar concentrations. Adenosine is released in BAT during stimulation of sympathetic nerves as well as from brown adipocytes. The adenosine A_2A receptor is the most abundant adenosine receptor in human and murine BAT. Pharmacological blockade or genetic loss of A_2A receptors in mice causes a decrease in BAT-dependent thermogenesis, whereas treatment with A_2A agonists significantly increases energy expenditure. Moreover, pharmacological stimulation of A_2A receptors or injection of lentiviral vectors expressing the A_2A receptor into white fat induces brown-like cells—so-called beige adipocytes. Importantly, mice fed a high-fat diet and treated with an A_2A agonist are leaner with improved glucose tolerance. Taken together, our results demonstrate that adenosine–A_2A signalling plays an unexpected physiological role in sympathetic BAT activation and protects mice from diet-induced obesity. Those findings reveal new possibilities for developing novel obesity therapies.