<p>Nitrogen (N)-fixing species are widely used in forestation and agriculture. The effects of planting N-fixing species on soil organic carbon (SOC) stock, however, remain uncertain, limiting policy development and their application towards a possible climate change mitigation strategy. Here we conduct a global meta-analysis of 385 datapoints from 136 studies comparing SOC stock with planting N-fixing versus non-N-fixing species. Planting N-fixing species increases SOC stock by 16% compared with non-N-fixing species. This SOC increase is closely accompanied by soil N increases, with an average accumulation of 7.8 g of SOC per gram of soil N increase. Climate mediates SOC responses, with greater SOC sequestration observed in drier and warmer regions, particularly in the tropics. We estimate that an additional increase of 0.29–0.75 PgC yr⁻¹ in global SOC stock could be achieved by adopting N-fixing species for forestation, agriculture and regeneration of marginal lands, highlighting their potential for climate change mitigation.</p>