Significance We discovered that the tropical oceanic diversity depression is not a recent phenomenon nor very deep time in origin by using a comprehensive global dataset of the calcified shells of planktonic foraminifers, abundant unicellular organisms in the world's oceans, which are exceptionally well preserved in marine sediments as fossils. The diversity decline in the lowest latitudes may have started due to rapid post–ice-age warming around 15,000 y ago. Warming may by the end of this century diminish tropical oceanic diversity to an unprecedented level in human history.