Worldwide, chronic overfishing has depleted populations of large predatory reef fishes and caused unexpected, top-down changes in coral reef ecosystems. Groupers are especially susceptible to overexploitation, because they aggregate to reproduce at specific locations and times. An understanding of the spatial dynamics of these fishes is critical for fisheries management and conservation. However, movements and migration dynamics of endangered reef fishes are poorly known. We show, using acoustic telemetry, that Nassau groupers Epinephelus striatus exhibit highly synchronised migration to spawning sites, despite their otherwise solitary habits. Reproductive adults leave their individual territories in shallow waters near the winter full moons, and migrate to the same spawning site up to 4 times yr–1. At the spawning site, a remarkable population-wide depth change occurs within an hour as individuals dive to a maximum depth of 255 m. Our results greatly expand the previously known migration frequency and depth range of this species, and reveal an unexpected yet predictable complexity of adult fish migration between habitats. Effective conservation of this threatened species requires that deeper reefs and the timing of migration events be incorporated into fisheries management plans.