The principle hyperglycaemic hormone, glucagon, is secreted from pancreatic islet α-cells as part of the counter-regulatory response to hypoglycaemia. Hence, secretory output from α-cells is under high demand in conditions of low energy supply (glucose). Many tissues oxidise fat as an alternate energy substrate. Here, we show that glucagon secretion in low glucose conditions is maintained by fatty acid oxidation in both mouse and human islets, and that inhibiting this metabolic pathway profoundly decreases glucagon output (~40%) by depolarising α-cell membrane potential and decreasing action potential height. We demonstrate that this is not mediated by the KATP channel, but by using experimental and computational approaches, can instead be attributed to reduced energy supply to the Na+-K+ pump. These data suggest that counter-regulatory secretion of glucagon is driven by fatty acid metabolism, and that the Na+-K+ pump is an important ATP-dependent regulator of α-cell function.