Far more species of organisms are found in the tropics than in temperate and polar regions, but the evolutionary and ecological causes of this pattern remain controversial(1,2). Tropical marine fish communities are much more diverse than cold-water fish communities found at higher latitudes(3,4), and several explanations for this latitudinal diversity gradient propose that warm reef environments serve as evolutionary `hotspots' for species formation(5-8). Here we test the relationship between latitude, species richness and speciation rate across marine fishes. We assembled a time-calibrated phylogeny of all ray-finned fishes (31,526 tips, of which 11,638 had genetic data) and used this framework to describe the spatial dynamics of speciation in the marine realm. We show that the fastest rates of speciation occur in species-poor regions outside the tropics, and that high-latitude fish lineages form new species at much faster rates than their tropical counterparts. High rates of speciation occur in geographical regions that are characterized by low surface temperatures and high endemism. Our results reject a broad class of mechanisms under which the tropics serve as an evolutionary cradle for marine fish diversity and raise new questions about why the coldest oceans on Earth are present-day hotspots of species formation.