Ecosystem functioning is mediated by species’ traits. Functional diversity, defined as variation in the traits of populations, provides functional information overlooked by taxonomic measures. Studies examining the functional diversity of coastal fishes found variation in functional diversity to be a better indicator of resilience. However, the mechanisms linking changes in biodiversity to variation in ecosystem functioning are poorly understood because: (1) until recently, we lacked empirical data on the morphological traits of fish assemblages, and (2) uncertainty surrounds the number and type of metrics to employ. We provided the first comprehensive global review on the functional diversity of coastal fish assemblages (n = 58 studies) and assessed the predictive performance of 36 metrics. The best performing metrics across all seascapes were multidimensional space metrics. We quantified four multidimensional space indices to identify functional linkages between the fish and coral communities of Lady Elliot Island, Eastern Australia. Coral functional evenness was a significant predictor of fish functional diversity. Fish functional richness, evenness, and divergence increased with coral functional evenness. Our findings highlight, for the first time, that fish functional diversity is mediated by coral functional diversity, with implications for management and restoration strategies aimed at supporting ecosystem functioning and reef recovery.