The Laacher See Eruption (LSE) ranks among Europe’s largest Upper Pleistocene volcanic events. Although its tephra deposits represent an important isochron for the synchronization of proxy archives at the Late Glacial to early Holocene transition, uncertainty in the eruption age has prevailed. Combined analysis of high-precision ring width and radiocarbon measurements from individual rings of trees that were killed during volcanic eruptions and buried by their deposits can provide eruption dates with annual and even sub-annual resolution. Here, we present dendrochronological and radiocarbon measurements of subfossil trees buried by pyroclastic deposits that firmly date the LSE to 13,006 ± 9 cal BP, more than a century earlier than hitherto accepted. The revised age of the LSE, now the oldest volcanic eruption worldwide dated with such precision, necessarily shifts the chronology of European varved lakes relative to the Greenland ice core record, thereby dating the onset of the Younger Dryas at 12,807 ± 12 cal BP, around 130 years earlier than thought. Our results synchronize the Younger Dryas onset across the North Atlantic-European sector, preclude a direct link between the LSE and Greenland Stadial-1 cooling, and suggest a large-scale common mechanism of a weakened Atlantic Meridional Overturning Circulation under warming conditions.