<jats:title>Significance</jats:title><jats:p>We use distribution data on 48,661 species to show that marine biodiversity has been responding to climate warming at a global scale. We show that marine species richness levels off or declines in latitudinal bands with average annual sea surface temperatures exceeding 20 °C. This results in a dip in species richness around the equator that has become more pronounced as the climate has warmed, especially for pelagic species. Previous studies have either only predicted such effects or have provided data at regional scales or for limited numbers of taxa.</jats:p>