During the Quaternary period, organic-rich black layers called sapropels were intermittently deposited in the deep eastern Mediterranean Sea(1,2) following high flood periods of the Nile River(3). During the past 250 kyr, timing of sapropel formation coincides with astronomically driven maximum summer insolation in the northern tropics(4). The insolation variations-described by a monsoon index(4)-modulate the intensity of the African monsoon feeding the Nile flood(4). Here, we report the observation of a thick sapropel in eastern Mediterranean sediments that conspicuously deviates from the usual pattern. The sapropel, dated at 528-525 kyr by astronomical tuning of the stratigraphic oxygen-isotopic record, is anomalous because the tropical summer insolation, while at a peak at this time, was much lower than during the deposition of the more recent sapropels. The Mediterranean climate was cold and dry, at least at sea level At the same time, in the equatorial Indian Ocean there was an extreme event of low surface-water salinity caused by heavy monsoonal fluvial discharge(5). The simultaneity, within dating. uncertainties, of unusually heavy monsoon rainfall over Africa and Asia while summer insolation (and the monsoon index) was relatively low indicates a large, regional-scale monsoon anomaly that cannot be explained in terms of current understanding of astronomical forcing.