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Background and aims: Legume N2 fixation is highly sensitive to drought. Elevated [CO2] (e[CO2]) decreases stomatal conductance (gs) and improves water use efficiency (WUE), which may result in soil water conservation and allow N2 fixation to continue longer under drought. Using a Free-Air CO2 Enrichment (FACE) approach, this study aimed to elucidate whether e[CO2] improves N2 fixation of Pisum sativum L. under drought.Methods: In a FACE system, plants were grown in ambient [CO2] (~400 ppm) or e[CO2] (~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 N2 fixation.Results: Lower gs under e[CO2] increased water use efficiency at leaf and plant level, and this translated to slower soil water depletion during drought. Elevated [CO2] increased WSC and decreased total AA concentrations in nodules, and increased nodule activity under drought. N2 fixation was stimulated (+51%) by e[CO2] in proportion to biomass changes. Under e[CO2] a greater proportion of plant total N was derived from fixed N2 and a smaller proportion from soil N uptake compared to a[CO2].Conclusion: This study suggests that e[CO2] 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 N2 fixation. Our results suggest that growth under e[CO2] can mitigate drought effects on N2 fixation and reduce dependency on soil N resources especially in water-limited agro-ecosystems. [ABSTRACT FROM AUTHOR]