Background and aims Legume N 2 fixation is highly sensitive to drought. Elevated CO 2 (eCO 2 ) decreases stomatal conductance (g s ) and improves water use efficiency (WUE), which may result in soil water conservation and allow N 2 fixation to continue longer under drought. Using a Free-Air CO 2 Enrichment (FACE) approach, this study aimed to elucidate whether eCO 2 improves N 2 fixation of Pisum sativum L. under drought. Methods In a FACE system, plants were grown in ambient CO 2 (~400 ppm) or eCO 2 (~550 ppm) and subjected to either terminal drought or well-watered treatments. Measurements were taken of photosynthesis, soil water dynamics, water soluble carbohydrates (WSC), amino acids (AA) and N 2 fixation. Results Lower g s under eCO 2 increased water use efficiency at leaf and plant level, and this translated to slower soil water depletion during drought. Elevated CO 2 increased WSC and decreased total AA concentrations in nodules, and increased nodule activity under drought. N 2 fixation was stimulated (+51%) by eCO 2 in proportion to biomass changes. Under eCO 2 a greater proportion of plant total N was derived from fixed N 2 and a smaller proportion from soil N uptake compared to aCO 2 . Conclusion This study suggests that eCO 2 increased WUE and this resulted in slower soil water depletion, allowing pea plants to maintain greater nodule activity under drought and resulting in appreciable increases in N 2 fixation. Our results suggest that growth under eCO 2 can mitigate drought effects on N 2 fixation and reduce dependency on soil N resources especially in water-limited agro-ecosystems.