Human-induced global changes such as nitrogen (N) deposition, climatic warming and rainfall changes have been determined to be common drivers of current plant community dynamics. However, it is unclear if and how the individual and combined effects of these drivers differently influence plant diversity and its relationship with productivity at the global scale. Here, we performed meta-analyses with data compiled from 133 articles, comprising >2000 effect sizes, to assess the individual and combined effects of N addition, warming and increased rainfall on plant diversity and its relationship with productivity (using aboveground biomass as a proxy). We found that N addition decreased species richness, Shannon–Wiener index (H′) and evenness, while it increased aboveground biomass. In contrast, warming and increased rainfall had no effect on diversity although both also increased aboveground biomass. The combined effects of N addition + warming and N addition + increased rainfall showed significant negative effects on plant diversity, with additive and synergistic interactions, respectively. Warming + increased rainfall did not influence plant diversity. Both the individual and combined effects on plant diversity were influenced by several moderator variables, with negative impacts of the magnitude and experimental duration on N addition effects and of latitude on N addition + warming effects. Importantly, our results showed that the greater the increase in plant productivity with long-term N addition, the greater the decline in plant diversity, and vice versa, indicating that the commonly observed positive diversity–productivity relationship would be reduced under long-term N addition. Our study provides new insights for the development of predictive models of plant diversity dynamics in response to multiple concurrent global change drivers, while also highlighting a consistent, strong negative effect of N addition, pointing to a clear need for reducing N deposition.