Forests are recognized as the largest natural source of nitrous oxide (NO) emissions on land, with deforestation drastically reducing the cover and biodiversity of native forests worldwide. Yet, how losses in forest biodiversity affect soil NO fluxes remains poorly understood. Here, we combined a global tree diversity-forest soil NO data set, including 201 paired comparable observations from global forests, with a three-year field survey of flux data gathered from a long-term plant diversity field experiment. Our analyses reveal that tree diversity has a significant negative effect on soil NO emissions, primarily driven by a decrease in NO production associated with denitrification. More specifically, we showed that reductions in NO emissions with tree diversity can be attributed to a decrease in the availability of soil inorganic nitrogen. Predictive modeling further shows that compared to forests with a single tree species, forests with two tree species can reduce global forest NO emissions by 10.39%, while those with 24 tree species achieve the maximum mitigation effect, reducing emissions by 56.30%. Taken together, our work highlights the contribution of tree diversity for mitigating NO emissions, highlighting the importance of accounting for biodiversity when reforesting old forests and supporting new afforestation processes.