The mortality rates of large trees are critical to determining carbon stocks in tropical forests, but the mechanisms of tropical tree mortality remain poorly understood. Lightning strikes thousands of tropical trees every day, but is commonly assumed to be a minor agent of tree mortality in most tropical forests. We use the first systematic quantification of lightning-caused mortality to show that lightning is a major cause of death for the largest trees in an old-growth lowland forest in Panama. A novel lightning strike location system together with field surveys of strike sites revealed that, on average, each strike directly kills 3.5 trees (>10 cm diameter)and damages 11.4 more. Given lightning frequencydata from the Earth Networks Total Lightning Networkand historical total tree mortality rates for this site, we conclude that lightning accounts for 40.5% of the mortality of large trees (>60 cm diameter) in the short termand likely contributes to an additional 9.0% of large tree deaths over the long term. Any changes in cloud-to-ground lightning frequency due to climatic change will alter tree mortality rates; projected 25-50% increases in lightning frequency would increase large tree mortality rates in this forest by 9-18%. The results of this study indicate that lightning plays a criticaland previously underestimated role in tropical forest dynamics and carbon cycling.