Canopy structural diversity varies systematically with climate and species diversity across boreal, temperate, and tropical biomes. Yet, how this latitudinal variation affects forest productivity—particularly the role of structural diversity in mediating effects of climate on productivity across biomes—remains unresolved. By synthesizing airborne laser scanning data and ground-based forest inventories spanning boreal to tropical forests, we show that beyond its direct effects on forest productivity, climate influences productivity indirectly by regulating canopy structural diversity—a significant, yet previously underappreciated mechanism whose precise magnitude is challenging to isolate. Notably, we observe a pronounced latitudinal congruence between hotspots of structural diversity and productivity. This positions structural diversity not only as a complementary indicator of productivity but also as a critical mediator of the influence of climate and species diversity, essential for improved forecasting.