Background/Question/Methods Litter decomposition is key to carbon (C) and nutrient cycling in terrestrial ecosystems, and a vital link between above- and belowground biota and ecological processes. With rapid biodiversity decline due to anthropogenically initiated environmental degradation, there are increasing concerns as to how the reduction in plant species diversity may impact key ecological functions, including litter decomposition. While previous syntheses have suggested that litter mixtures generally decompose more quickly than monocultures, uncertainties in plant diversity–decomposition relationships, with various outcomes recorded to date, complicate predictions of plant diversity loss impacts on element cycling, such as the amount of CO2 released to the atmosphere, as well as soil biota. Importantly, it is unclear how biotic and climatic factors may drive the directions and magnitudes of plant diversity effects on litter decomposition. We aimed to identify the biotic and climatic factors involved in the various outcomes of plant diversity–decomposition relationships across terrestrial ecosystems, based on a meta-analysis of 492 paired observations of decomposition rates of leaf litter mixtures and corresponding monocultures from 110 studies. Results/Conclusions Litter mixtures on average increased litter decomposition rates by 5.6% (95% confidence intervals, 3.0–8.1%), and the effects of litter mixtures increased with litter species richness, the functional diversity of chemical traits (leaf C, N, P contents and C : N ratio) and phylogenetic diversity consistently across terrestrial ecosystems. The decomposer abundance and function, including soil fauna abundance, microbial biomass, and extracellular enzyme activities, were positively associated with litter mixture effects on decomposition rates. The structural equation models accounted for 48.6% of the global variation in litter decomposition rates and revealed that the positive effects of litter functional diversity on decomposer abundance and function led to increased litter decomposition rates, while litter phylogenetic diversity had a positive direct effect on litter decomposition rates. As indicators for complementarity effects, functional diversity of the chemical traits and phylogenetic diversity lead to more positive litter mixture effects on decomposition. The positive litter diversity effects on decomposition rates are mechanistically linked with soil fauna abundance, microbial biomass, and extracellular enzyme activities. Our results suggest that plant diversity, especially functional and phylogenetic diversity, increases decomposer abundance and function, and thus plays a key role in carbon and nutrient cycling across terrestrial ecosystems.