Environmental heterogeneity has long been assumed to promote biodiversity, but recent theory and evidence suggests substantial variability in this relationship. Here, we resolve heterogeneity’s influence on communities by considering its various facets and contextual moderators, capitalising on >3000 effects reported in 144 studies on rocky reefs worldwide. Our analyses reveal generally positive saturating or weakly humped-shaped responses of richness and abundance, with three-dimensional structure generating the strongest effects of all facets. Small-bodied and swimming organisms benefitted the most and larger-bodied benthic organisms the least, while heterogeneity also mitigated grazing and enhanced recruitment. Community responses strengthened on biogenic vs. abiotic substrates, and with environmental stress, both towards the equator and on intertidal vs. subtidal reefs. We propose that the key moderators unveiled here (heterogeneity facet, benefactor traits, and environmental stress) will more generally underpin context-dependent effects of heterogeneity on biodiversity across organismal groups and ecosystems.