The Younger Dryas (12.9 ± 0.1 to 11.7 ± 0.1 ka) was a return to cold conditions in the Northern Hemisphere during the last deglaciation. This climatic event was hypothesized to have been caused by a change in glacial Lake Agassiz (north-central North America) overflow from its routing to the Gulf of Mexico to an easterly route to the North Atlantic due to Laurentide ice-sheet retreat from the Lake Superior basin, which caused a reduction in Atlantic meridional overturning circulation. Alternative models argue that Lake Agassiz triggered the Younger Dryas via northwestward routing to the Arctic Ocean. We present new 10Be surface exposure ages that directly date ice retreat from eastern Lake Agassiz outlets and show that the area was ice free at the onset of the Younger Dryas. The southernmost eastern channels opened at 14.0 ± 0.4 ka and 13.6 ± 0.2 ka, but an ice-free route through the Lake Superior basin only opened after 13.5 ± 0.5 ka. The main eastern channel to the eastern Great Lakes and North Atlantic opened at 13.0 ± 0.1 ka to 12.7 ± 0.3 ka. This channel opening was concurrent with decreased runoff to the Gulf of Mexico and increased runoff through the lower Great Lakes to the Gulf of St. Lawrence and North Atlantic. Gulf of St. Lawrence runoff records and isostatic-rebound modeling suggest eastern outlet abandonment at ca. 12.2 ka, with possible northwestward routing of runoff. Our results confirm that Lake Agassiz overflow could have been routed eastward to the North Atlantic at the Younger Dryas onset and caused the canonical abrupt climate change event.