Phosphorus is frequently the limiting nutrient in marine and terrestrial ecosystems, largely owing to its poor solubility and slow movement through the rock cycle. Phosphorus is viewed to exist in geological systems almost exclusively in its fully oxidized state as orthophosphate. However, many microorganisms use the partially oxidized forms-phosphite and hypophosphite-as alternative phosphorus sources, and genomic evidence suggests that this selectivity is ancient. Elucidating the processes that reduce phosphate is therefore key to understanding the biological cycling of phosphorus. Here we show that cloud-to-ground lightning reduces phosphate in lightning-derived glass compounds, termed fulgurites. We analysed the phosphorus chemistry of ten fulgurites retrieved from North America, Africa and Australia, using microprobes and 31P nuclear magnetic resonance. Half of the fulgurites contained reduced phosphorus, mainly in the form of phosphite. The amount and type of reduced phosphorus was dependent on the composition of the fulgurite section examined: carbon-rich sections contained around 22% reduced phosphorus in the form of iron phosphide, whereas other fulgurites contained between 37 and 68% in the form of phosphite. We suggest that lightning provides some portion of the reduced phosphorus used by microbes, and that phosphate reduction by lightning can be locally important to phosphorus biogeochemistry.