Despite increasing evidence suggesting mixed-species plantations promote biodiversity, a comprehensive quantitative analysis of this knowledge is lacking. We systematically reviewed 71 studies to evaluate the effects of mixed versus pure tree plantations on biodiversity. Using descriptive statistics and meta-analyses, we explored: a) the effects of mixed plantations on forest-related biodiversity; b) variations in these effects with climate, stand age, and with the richness, relative abundance, and functional diversity of the planted species; and c) differences in responses across taxonomic and functional groups. Our meta-analyses revealed a significant positive effect of mixed-species plantations on taxonomic diversity. However, most observations (64%) reported no significant effects. Positive effects are more frequent in mixtures with more than two species (49%), compared to two-species mixtures (29%), and were strongest in tropical climates (78%), followed by temperate (26%) and continental climates (14%). Among taxonomic and functional groups, positive mixing effects are most frequent for birds (75%), followed by litter microbiota (47%), understory plants (40%), and above-ground arthropods (29%), while soil-dwelling micro-organisms (22%) and soil mesofauna (4%) appear less sensitive. Mixing conifers and broadleaves does not enhance biodiversity benefits, suggesting higher functional diversity may be better achieved by targeting specific species and traits. The limited effects of mixing observed in some cases may reflect the young age of plantations studied (11±9.4 years on average), which may limit the time for biodiversity to respond. The variability in biodiversity outcomes highlights the need for tailored mixing strategies and further research across broader plantation ages, settings, and underrepresented taxonomic groups to optimize biodiversity benefits in mixed-species plantations.